I think the geographic/geologic diversity of the United States is fascinating, and I wish I had learned more about it before I started traveling around the country. I have been reviewing information on the physical regions of the lower 48 states, and I want to share this with you. My text is fleshed out with links to the USGS (United States Geological Survey), the National Park Service, and other good sources.

A discussion of landforms must include some basic geological lingo, but you will pick it up as we go. However, when you would like a definition of a term, go to Alphabetical Geologic Glossary.

GEOLOGIC TIME

In Hendrik Willem Van Loon's children's classic, The Story of Mankind, he wrote this:

High up in the North in the land called Svithjod, there stands a rock. It is a hundred miles high and a hundred miles wide. Once every thousand years a little bird comes to this rock to sharpen its beak. When the rock has thus been worn away, then a single day of eternity will have gone by.

To appreciate geological processes and the evolution of American landscapes, we don't need to grasp eternity, but we do need to get a feel for the almost unfathomable reach of geologic time. Where we chronicle our lives in decades, years, months, and days, geologic time is recorded in eons, eras, periods, epochs, and ages.

The first fixed date in our historical record is 4241 BC when the Egyptian calendar regulated by the Sun and Moon was established. That was about 3.3 billion minutes ago. The Earth was formed about 4.6 billion years ago. The 4 in the 4.6 BY (billion years) represent the Cryptozoic (hidden life) Eon (also commonly called Precambrian time) in which there were very few organisms to leave hard parts for the fossil record. You can find Precambrian rock in such places as the bottom of the Grand Canyon.

The .6 in the 4.6 BY life of Earth or about 550 MY represents the Phanerozoic (visible life) Eon that is divided into these three eras:

• The Paleozoic (old life) Era (550 to 250 MYA [million years ago]) saw an astonishing explosion of life forms at the beginning of the era in the Cambrian Period. Invertebrates, fish, insects and, finally, land plants and reptiles evolved during the era. However, it ended with the catastrophic Permian mass extinction. In the Late Paleozoic the continents assembled to form a supercontinent (Wegener's Pangea). The limestone in top layer in the Grand Canyon, the Kaibab Formation, was deposited in a shallow sea near the close of this era.

• The Mesozoic (middle life) Era (250 to 65 MYA) was the age of dinosaurs, birds, and first small mammals. A big story in the Mesozoic was the breakup of the supercontinent, Pangea. North America and Europe were closer to the equator. It ended with the Cretaceous mass extinction, which ended the long reign of the dinosaurs. The prevailing theory is that this extinction was caused by a meteor that collided with Earth.

• The Cenozoic (recent life) Era (65 MYA to present) is the age of mammals, who found open niches after the demise of the dinosaurs. The dinosaur's loss was our gain because we probably would not have evolved had it not been for the subsequent explosion in mammal evolution. The final separation of the continents was completed, and most of the landforms we will explore in our tour of America were either formed or were profoundly changed in Cenozoic times. The Cenozoic is divided into the Tertiary Period (that is further divided into the Paleocene starting 65 MYA, Eocene, Oligocene, Miocene, and Pliocene Epochs) and the Quarternary (that is divided into the icy Pleistocene starting 2 MYA, and Holocene Epochs).

To see good representations of the geologic time scale, go to Geologic Time Scale or for more detail go to Web Geological Time Machine. For a good discussion of geologic time go to Fossils, Rock, and Time and to Geologic Time, which includes a page on index fossils.

Time units on the geologic time scale were based on the appearance and disappearance of fossils. For example, the marine deposits of Devonshire, England, with their particular fossils were the marker for what was named the Devonian Period of the Paleozoic Era. All of the deposits during the Devonian Period of time make up the Devonian System of rocks.

Prior to the 20th Century the geologic time scale was only a relative time scale. For example, the rocks of the Devonian System were deposited on top of rocks of the Silurian System and therefore should normally be younger than the latter, but the geologists didn't know the absolute age of either system. Absolute dating of rocks was not possible until radioactive decay was understood and put to use in paleochronology. We now know that Devonian time, when the first amphibians and trees appeared, is the period from 360 to 408 MYA.

By examining rock outcrops, road cuts, and well drilling records geologists map formations, which are identifiable and mapable rock units or bodies of rock. For example, in the Ithaca area of New York, field geologists found a unit of rock that physically differed from other units, so they deemed it to be a distinct unit and named it the Ithaca formation. A rock unit is based entirely on physical characteristics without reference to time. It is satisfying to be able to identify dominant formations in our home areas.
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THE DYNAMIC EARTH

From the perspective of our short lives our continent seems virtually unchanging. However, there are geological processes acting all around us constantly building, degrading, and recycling the Earth's crust. Every mountain and even every ocean is a transient feature. The marine limestone rocks at the top of Mt. Everest were formed from limey sediments at the bottom of the sea at a location that might have been thousands of miles away. Change can come fast. The Himalayas are young mountains that started forming as a result of a continental "collision" less than 50 MYA, and they are still growing higher at the rate of about 2.4 inches per year, which is a mile in only 26,400 years. The Atlantic Ocean is only about 200 MY old on an Earth that is 4.6 BY old.

An astonishing change in our continent over the millions of years is how it grew in size. We are not only a nation of immigrant people but also a country of many "immigrant" (exotic) terranes (pieces of the Earth's crust such as minicontinents and islands) which came to our shores by piggybacking on tectonic plates. Assuming that an island arc like Japan moved 3,000 miles to our shores at the rate of about an inch per year, it would take 190 MY for the trip, which is only a bit longer than the Mesozoic Era.

Earth's dynamism is powered by three great engines: 1) gravity, 2) the heat of the Sun, and 3) the Earth's internal heat.

Gravity has a large role in erosion and deposition. It brings material down to lower elevations. It may do it directly through dramatic landslides and unnoticeable soil creep or with the help of water running down hill, transporting grinding rocks and abrasive sand, and carrying away the debris.

Gravity powers vertical movement of the crust through the principle of buoyancy or isostacy. A mountain rides higher or lower on the taffy-like layer of the mantle just as a boat rides higher of lower depending upon how heavily it is loaded. If a mountain loses some of it mass by erosion, it rises, and the site of deposition sinks. The Hudson Bay was depressed by the heavy load of ice during the Ice Age. Now it is gradually rebounding, and someday the bay may disappear.

The Sun drives the weather including wind and the water cycle of evaporation and precipitation, which results in erosion. It also drives physical weathering from heating/cooling and freezing/thawing. Snow can pile up and form mountain glaciers or even continental glaciers that bulldoze landscapes.

The Sun makes vegetation possible because it provides the energy for photosynthesis. Vegetation slows erosion, but its roots may increase mechanical weathering and acids from its decomposition increases chemical weathering.

The Earth's internal heat causes convection currents in the mantle. This drives the tectonic cycle, which includes the process of ocean floor spreading and destruction and the process of mountain building. (Tectonics is the study of the movement and deformation of the Earth's crust. This includes faulting, folding, warping, uplifting, and plate movement.) An understanding of the tectonic cycle is to geology as an understanding of evolution is to biology.

A PERSONAL STORY In 1943 I was in the Army Specialized Training Program at the City College of New York. One of our courses was geography taught by Professor Hastings, who was a legendary figure at City College. One day he made a passing reference to the theory of drifting continents, which was championed by Alfred Wegener, a German meteorologist who in 1915 wrote The Origin of Oceans and Continents. It was as controversial as Darwin's Origin of the Species in 1859. Wegener held that there was once a single supercontinent, which he called Pangea, that separated into the existing continents that drifted apart. This extraordinary notion piqued our curiosity, but Hastings was reluctant to say more---probably because it was blatant geological heresy. However, we persisted, and he ticked off the evidence that supported the theory. Anyone who has ever compared the two coastlines on either side of the Atlantic Ocean may well have wondered if the fit is something more than coincidence. Although there was strong evidence for the theory, most geologists thought the notion of continents "drifting" over the ocean bottoms was absurd. In 1962 I went back to graduate school and took courses in geology on the side, but I got there about five years too soon. While I was studying the old geology that kept continents firmly anchored in place, a fantastic new geology was rapidly gestating.

By 1967 the crustal movement model was worked out and became known as plate tectonics. It wasn't long before the boundaries of seven major and several minor tectonic plates were mapped, and the different types of plate boundaries were recognized. Since the evidence was so compelling, most geologists soon accepted the theory and started rewriting the geology books. There may still be a few holdouts, but there may also be a few "flat-earthers."

The fascinating story of how geoscientists so neatly solved the problem of how continents move and how they so convincingly confirmed it with different lines of evidence is one of the great stories of science. To read all about it, go to Developing the theory.

The Action at Plate Boundaries

The continental margin on our east coast is now a passive margin, but that was by no means always the case. The west coast has a more recent history of accretions of land to the coast, and continuing earthquakes and volcanoes. Now we know why it is an active margin.

Continental plates converge with oceanic plates all along the west coast of North and South America and along the east coast of Asia. This boundary around the Pacific rim is called the Ring of Fire because of the frequent volcanoes and earthquakes. Where the heavier oceanic plates are forced under the lighter continental plates, we get volcanoes such as those in the Cascades. Where the plates grind past each other (as they do in California), we get earthquakes and slippage of landscapes along the faults. Where continental plates collide head on without subduction, the crust is folded and thrusted resulting in mountains such as the Appalachians (as well as the Ural Mountains, the Himalayas, and the Alps). Two converging oceanic plates create an island arc such as the Aleutians and Japan.

The nature of a volcano depends upon its type and the chemical composition of the upwelling magma. There are four types of volcanoes with respect to their source: 1) Hot-spot volcanoes where there is a hot-spot in the mantle that melts through the crust. Fine examples are the Hawaiian shield volcanoes. They are awesome to watch but not explosive because they produce base-rich lavas coming from the mantle that flow freely on the surface. 2) Stratovolcanoes occur when an ocean plate dives down into a deep ocean trench and slips below a continent (or subducts) as it does in the Pacific Northwest. The plate carries down continental sediments that are melted forming lighter silica-rich magmas that rise like cream and are sticky and explosive when they feed stratovolcanoes such as Mt. St. Helens. They build up cones and produce huge amounts of volcanic ash. 3) Oceanic arc volcanoes where one oceanic plate slips under another oceanic plate. The Aleutian Islands are a good example. 4) Oceanic ridge volcanoes occur along mid-ocean ridges where the oceanic plates are diverging. They usually occur in deep water, but in the case of Iceland they occur on land.

For some excellent background information on the earth's structure and on plate tectonics go to The Restless Earth: A Geologic Primer. Also go to This Dynamic Earth, where I have already cited "Developing the theory."
Back to the Beginning

ROCK TYPE AND STRUCTURE

For an example of how rock type and structure control the processes of weathering and erosion that sculpt landforms, let's go to the Sierra Nevada. When an oceanic plate was slipping below the North American plate, the country rock to the east was intruded by magma welling up from the mantle below. Rock that is formed from the cooling of magma is called igneous rock (from ignis meaning fire). The magma that moved up and cooled below the surface in large balloon-like masses called plutons, formed rock called, no surprise, plutonic rock. The whole collection of plutons is called a batholith, hence the Sierra Nevada Batholith. The rising magma consisted of various compounds, and as it slowly cooled, the compounds were differentiated into mineral crystals, mostly feldspars, quartz, and mica. Since the magma cooled slowly, the crystals grew so large that they are easy to see in these granitic rocks. The magma extruded on the surface formed a volcanic igneous rock. It cooled so rapidly that the mineral crystals are so small that we can't see them with the naked eye.

Have you seen a kitchen counter made out of granite? It is one hard, tough rock, but with enough time it is no match for the relentless geologic processes of physical and chemical weathering and erosion from running water, wind, and ice. Where granitic rock is exposed in the Sierras, physical weathering from freezing and thawing exfoliates it making rounded domes such as Half Dome before it was cleaved by a glacier. Joints (cracks caused by regional or local tensions) are invaded by water. Chemical weathering attacks the minerals in the rock, especially the feldspars. Granite is converted to sand (from the quartz grains) and to clay (from the feldspar). In places you find banks of "decomposed granite" in which the granitic rock is so weathered that you can dig into it with a shovel.

The sand, silt, and clay was eroded from the Sierras, transported by streams, and deposited in the Great Central Valley. As the stream currents slowed, sand was the first to be deposited followed by silt and then clay in slack water. Thousands of feet of these sediments were built up and eventually lithified ("rockified") from the pressure and heat. Sand grains are cemented together to form sandstone, silt forms siltstone, clay forms shale, and carbonates form limestone, all of which are sedimentary rocks.

When the old sedimentary rock of the Sierra Nevada was exposed to the heat and pressure of the invading magma, some of the rock was changed to metamorphic rock. Quartz sandstone was changed to quartzite, shale to slate, and limestone to marble. When granite is exposed to enough heat and pressure, it is changed to gneiss /nice/. With still more heat and pressure metamorphic rock can be remelted and become igneous rock again. The way rocks can change from one type to another is often depicted as the rock cycle.

In addition to rock type a second rock characteristic that is very important in landform construction and destruction is rock structure. The sedimentary rocks are originally deposited in horizontal beds. Tectonic deformation produces structural features such as joints (cracks), faults, folds, thrusts, basins, and domes. Geologic structure is the architecture of a rock body.

An example of the role of structure in shaping landforms is the development of topography of the western slope of the Sierra Nevada. Faulting and tilting of the great block of rock to the west, resulted in streams flowing down hill to west. Stream erosion was aided by joints or cracks in the rock running east and west. Deep canyons such as Yosemite Valley resulted from the water and ice erosion.

To read more about rocks, go to The Rock Cycle(1). For more on rocks with great graphics go to Rocks and Minerals and click on the linked terms. To learn more about rock structure go to Structural Geology.
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LANDFORMS

When we travel across the country, we travel on or by a great variety of landforms of different scales. The scale ranges from the largest, our continent, to the smallest, which may be an individual sand dune. Here are four kinds of landforms based on the processes by which they were formed:

1. Structural landforms: These are created by earth movements and include folded and fault block mountains.
2. Erosional landforms: These are sculpted by water, wind, glaciers, and gravity to form mountain horns, rounded granite domes, hog-back ridges, stream incised plateaus, glacial valleys, etc.
3. Depositional landforms: Eroded material is deposited by water, ice, and wind to form flood plains, glacial till plains, sand dunes, etc.
4. Weathering landforms: The best example of these are sinkholes and karst topography resulting from water dissolving limestone. Florida's abundance of small, round lakes and sinkholes are examples.

Some landforms fall into more than one of these categories. For example, an Appalachian mountain is typically a structural fold that has been weathered and eroded.

For a good landform glossary go to Geology 101.

TEN PHYSICAL REGIONS OF THE U.S.

A textbook continent can be subdivided into three large scale landforms: (1) the craton or stable heartland consisting of a Precambrian shield (a very old worn down continental nucleus consisting mostly of granitic and metamorphic rock) and the platform (sediments deposited on the outer part of the shield), (2) an old mountain belt on the passive margin and a younger mountain belt along the active margin, and (3) the coastal plains and continental shelves along the passive margin. The North American continent fits this model except for the Rocky Mountain system. It is too far from the active margin to be neatly explained by the standard plate tectonic theory.

The old standard work on the division of the United States into physiographic regions was Nevin M. Fenneman's Physiography of the Western United States, 1931, and Physiography of the Eastern United States, 1938, which were my first sources on the subject and are among my present sources. A revision of Fenneman's map was published by the Geological Survey in 1946 that delineated eight "major divisions" and 25 "physiographic provinces" within the conterminous U.S. The physiographic provinces have also been called "natural regions," "geomorphic regions," and "natural landscapes." Different physiographers will categorize landforms somewhat differently, but their results are quite similar. Now we will take our tour zigzagging across the United States through all of the physiographic provinces.

For a relief map that delineates the physiographic divisions go to Geologic Provinces. Go down to the second map and click on the delineated areas to go to a brief description of the regions.

For a great artistic landform map of the United States go to US Landform by Erwin Raisz. You will especially appreciate this map after you have explored the physiographic provinces and can identify them on the map.

I make use of a fantastic tool for viewing landforms, which is TerraServer. You can view a topog map of areas at different resolutions to identify landmarks and fix your location and then switch to an aerial photo of the same area to check on the land use.

To see a map delineating all of 25 physiographic provinces and to see a list of them, go to the USGS map that also shows the topography and geologic ages of the bedrock in color (see the code at the bottom) Physiographic Regions of the United States.

To obtain more information about the geology of each state go to State Geological Surveys.

Before starting our tour I recommend reading the excellent Physiography of the Earth's Terrestrial Surface.

Now we will look at the ten divisions of physiographic provinces that we are going to visit on our tour across the lower 48 states.
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LAURENTIAN UPLAND

For the USGS map and brief description go to Laurentian Upland Province: Superior Upland.

The Laurentian Upland division consists of the Superior Upland province of northeastern Minnesota, northwestern Michigan, and northern Wisconsin. Why, you may well ask, would we start there? It is because this is the part of the North American Precambrian Shield (or Canadian Shield), which is the core of the continent that dips down into the U.S. (Some physiographers put the Superior Upland and the Adirondack province in the same division because they are both part of the shield.) The shield has been very stable since Precambrian time, but it had a hectic youth. The shield was built by the accretion of several "immigrant" terranes.

ZOOM IN To see a sample Laurentian Uplands landscape, go to the USGS Topo Map. Switch to the air photo by clicking the Aerial Photo tab above the upper right-hand corner of the topo map, and you will see that there is a surprising amount of cleared land in between the lakes and bogs.

The shield bedrock is Precambrian igneous and metamorphic rock that is up to nearly 4 B.Y. (billion years) old. The shield was thoroughly glaciated during the Ice Age and was left with only a thin layer of till covering much of the bedrock. The topography is generally gently rolling with a few worn-down mountains. There are abundant swamps and lakes (well known to pike and walleye fisherman) and other evidence of glaciation.

The Mesabi Range with its rich iron ore deposits are in the province. The iron ore containing hematite is interbedded with other Precambrain rock. One theory is that the iron was transported to an ocean basin with no oxidation because there was very little atmospheric oxygen at that time.

This region has a short growing season and hard winters. You may have heard the name Iron Mountain, Michigan, included in winter reports of the coldest spots in the nation. The vegetation is mostly conifer and hardwood forests.

INTERIOR PLAINS

For a USGS map and description go to Interior Plains Division. For a map showing the provinces and sections go to Interior Plains Map.

This division is the "heartland of America" physically as well as culturally. It is where the bulk of our wheat and feed crops are grown. To see how much of it is arable, go to Cropland as a Percent of Land Area.

The Interior Plains division consists of sedimentary plains on top of the outer North American Shield. The shield along with the sedimentary plains is the stable core of the continent that, as we have noted, is called the craton. The bedrock under the Central Lowlands and the basement rock under the Great Plains was formed mostly from sediments deposited in seas. The upper rock on the High Plains was formed from stream sediments on land.

Central Lowlands

This province borders the Great Plains on the west, extends as far north as the Canadian border, as far south as the middle of Texas and the Ozarks, and as far east as Ohio. It includes most of the Corn Belt, which you can see by going to Corn as Percent of Cropland.

ZOOM IN For a closer look at an area in the heart of the corn belt, go to the USGS Aerial Photo. This is just northeast of Ft. Dodge, whose airstrip is shown. The Des Moines River is on the west side. Increase the resolution and see the well-developed farmsteads that tell you that this is prime corn country.

The bedrock consists mostly of old Paleozoic sedimentary rock (sandstones, conglomerates, shales, and limestones deposited in a number of transient shallow seas over a period of a half-billion years) sitting on the shield basement rock. The rock tends to be horizontally bedded although there are some structural domes and basins.

The northern part of this region was substantially changed by at least four glacial "facelifts" during the Pleistocene "Ice Age" starting about 2 MYA and ending about 10,000 years ago. Continental glacial ice sheets gliding over all but the highest mountains are hard to imagine. Due to their great weight and load of abrasive rocks, they grind down everything in their path and transport some of the debris. The last glaciation in the Wisconsin Age cut and filled the landscape across the heart of the Corn Belt leaving behind a nice layer of till, which was good parent material for the soil that developed. The topography is generally gently undulating.

After the retreat of the last glacier, melt water formed Glacial Lake Agassiz, which was larger than all of the present Great Lakes put together. After a thousand years the lake retreated and left a very large, deep, flat, lake-laid, fertile plain in what is now the Red River Valley, which is a misnomer since it is a lake plain rather than a river valley. The plain is the flattest and youngest major landscape in the lower-48. It is so flat that much of it requires a network of drainage ditches and is subject to serious flooding. It is the most important spring wheat producing area and also produces potatoes and sugar beets. Oddly, the Red River of the North, which forms most of the boundary between ND and MN, flows to the north into Lake Winnipeg, a remnant of Lake Agassiz and a part of the Hudson Bay watershed. I-29 takes you from the Canadian border down (or up elevationwise) through the "valley" through Grand Forks to Fargo. The lake sediments are so deep that the large buildings in Fargo are built on piling that goes down about a hundred feet.

Eastern Minnesota is the Western Lakes section of the Central Lowlands. Minnesota has about 15,000 lakes as a result of its relatively recent glaciation. Lakes are geologically short-lived because they are either filled with sediments or drained by erosion of the outlet. South of the cold region boreal forests dairying takes over in southeastern MN and southern WI.

Oddly enough, southwestern Wisconsin escaped glaciation and is called the Driftless Area, which has more topographic relief than the glaciated areas. The southern Osage Plains covering parts of Kansas, Missouri, and Oklahoma were also not glaciated.

This province includes the Great Lakes section with its world-class scenic attraction, Niagara Falls. The top layer of rock on the falls is a hard dolomite limestone, but it is underlaid with soft shale and sandstone. When the softer rocks are eroded away, blocks of the dolomite break off. This has resulted in the falls retreating upstream by about seven miles. All waterfalls are temporary since there are eventually eroded away.

The climate in the province is humid with rainfall ranging from 20 inches in the west to 50 inches in the east. The rain falls mostly during the growing season, which partly explains why this is a leading agricultural region. It was originally the tall grass prairie.

Interior Low Plateaus

This province is a transitional region lying between the Central Lowlands and the Appalachian Plateau and covers central Kentucky and Tennessee. It has more relief than the former province and less than the latter. "Plateaus" is a bit deceiving since the elevation is about the same as that of the eastern Central Lowlands bordering on the north.

When you are driving west on Interstate 40, you leave the Appalachian Plateaus and enter the Interior Low Plateaus about 75 miles east of Nashville. In about 20 miles you drop from 2,000 feet elevation down to 1,000.

A bulge runs down through the center of the province called the Cincinnati Arch. On this arch there are two structural domes that have had their centers eroded down. This has left a ring of low hills or cuestas (low scarps) consisting of resistant rocks such as sandstones and around the domes. The northern dome, which is in Kentucky, is the Lexington Plain. The center of the dome is underlaid with limestone and is called the Inner Bluegrass region (around Lexington) which is highly prized by thoroughbred breeders for its nutritious grass and horsey milieu. The area is circled by a less desirable belt underlain by shale that in turn is circled by the Outer Bluegrass region. The southern dome is called the Nashville Basin because of the eroded center. In between the two domes is the Pennyroyal Plateau (locally known as the "Pennyrile" Plateau) in south central Kentucky. It is underlain by limestone and sinkholes and caves are common. Some surface streams disappear into sinkholes. This section includes the Mammoth Caves National Park. The plateau has been known for its burley tobacco.

ZOOM IN To see a Thoroughbred breeding farm in Bourbon County south of Paris, KY, on Stoney Point Road (a good address in the Inner Bluegrass region), go to the USGS Aerial Photo. To see the parallel, widely meandering streams unusually close together go to Topo Map. The contour lines reveal an interesting pattern of low rounded ridges sculpted by the streams. You can also see what appears to be some large sinkholes in underlying limestone.

Great Plains

There are few places where the boundary between large landforms is so abrupt as that between the Great Plains and the Rockies. The eastern boundary of this province runs down through middle N. Dakota, S. Dakota, Nebraska, Kansas, and takes in the Panhandles of Oklahoma and Texas. How would you recognize this boundary if you were driving west through the Central Lowlands? Well, some of this boundary is marked by noticeable east-facing scarps (cliffs). The boundary roughly coincides with the 100th Meridian, the 20-inch rainfall line, and the 2,000-foot contour. (The elevation rises about 10 feet per mile until the Plains abruptly meets the Rockies at about 5,000 feet.) This line also marks the boundary between the old short grass and tall grass prairies. Finally, it is the boundary that quite clearly defines where the West begins.

Another way to tell that you are in the Great Plains is to look at the soil in road cuts. You may see little nodules of calcium carbonate in the lower part of the soil profile. In some of the drier areas the calcium carbonate may form a hard layer called caliche. The soil moisture evaporates in the soil profile leaving the calcium carbonate. In the Central Lowlands there is enough moisture to percolate down through the soil profile and leach out the calcium. The calcium rich Great Plains soils are well-suited for their native short grasses.

A brief geologic story of the Great Plains is well told in the Geologic History of the Great Plains. Here is a summary of the story:

• For a half-billion years there were interior seas on the outer part of the North American Shield. Marine sediments up to two miles deep were deposited across the Interior Plains. The Great Plains was built on this platform.
• During the Late Cretaceous the Laramide orogeny began with the uplift of the present Rockies. Streams began eroding the mountains and depositing these Cretaceous sediments in a wide belt along the eastern slope.
• The Rockies continued to rise, and about 50 MYA volcanoes began spewing out ash that added to the sediments on the plains.
• After about 10 million years of stability there was more uplift of the Rockies and more volcanism. You can see some of these sediments in the South Dakota Badlands. Later the sediments were spread all over the Great Plains, and some of the remainder of these sediments is in the Ogallala Formation under the High Plain.
• About 5 or 10 MYA more western uplift caused the streams to change from deposition to erosion of parts of the Great Plains. The Tertiary sediments were stripped from much of the Plains, but the central High Plains was spared.

ZOOM IN When going west from Topeka, KN, on I-70, you enter the Plains Border section of the Great Plains near Hays, KN, whose longitude in West 99 degrees, whose annual precipitation is 23 inches, and whose elevation is about 2,000 feet. Just to the west of Hays the elevation rises about 150 feet, and the fields are cut up with eroded draws. See Aerial Photo.

Just as the Central Lowlands is corn country, the Great Plains is wheat country, Hard Red Spring wheat in the North and Hard Red (and White) Winter wheat in the middle and south. The hard wheats are high in gluten and are used for bread flour. To see the percentage of the land that produces wheat go to Wheat as a Percent of Cropland.

Although there is a certain sameness about the Great Plains, there are distinct differences that requires the recognition of many physiographic sections.

The northern-most section is the Missouri Plateau that is bounded on the east by the Missouri Escarpment, which rises about 500 feet above the Central Lowlands and runs diagonally from northwestern North Dakota southeast to southeastern South Dakota. Most of the more recent Tertiary mantle has been eroded away. Badland topography is common. The northern part of the plateau was glaciated, and most of the land is used to grow wheat. The southern part, southwest of the Missouri River, wasn't glaciated, and it has less than half of the land in crops, mostly wheat. The Missouri Plateau is separated from the High Plains to the south by the north-facing Pine Ridge Escarpment, which is 1,000 feet high at the maximum.

A few Rocky Mountain groups are outliers in the northern Great Plains. In Montana there are the Highwoods, Bear Paws, Little Rockies, and Big Snowy Mountains. One of the small mountain groups sticking up in the Missouri Plateau is the Black Hills section in southwestern South Dakota. It is a dome uplifted many millions of years ago. To appreciate the geology go to South Dakota Geology. Notice that the dome looks like an "eye" in southwestern corner of the state. Mt. Rushmore in the middle of the dome is granite that intruded a mica schist. When the magma cooled to rock, some cracks developed in the granite, and later these cracks were intruded with pegmatite, which can be seen as white streaks across the foreheads of Washington and Lincoln. (Also notice that these gentlemen are "aging" due to the weathering of the granite.)

Just beyond the northwest corner of the Black Hills, the extraordinary Devil's Tower stands about 850 feet above the prairie. It looks like the petrified stump of a gigantic tree. It is a volcanic remnant of such spectacular proportions that President Roosevelt made it the first National Monument. To learn more about this extraordinary landmark, go to Devil's Tower, Wyoming.

To the east of the Black Hills are the Badlands where erosion has gone amuck. For an interesting account of their geology go to Geology, Badlands National Park. For some great pictures of the Badlands and the Black Hills go to Badlands, Black Hills.

High Plains

The central section of the province is the High Plains section whose Tertiary surface sediments once covered all of the Great Plains and rose gradually up to the summits of the Rockies. Then erosion stripped off the Tertiary surface on the west in the Colorado Piedmont, on the north on the Missouri Plateau leaving the north-facing Pine Ridge Escarpment in North Dakota, and on the east in the Plains Border section. This left the High Plains as a low plateau in the middle. In the south the High Plains landform is known as the Llano Estacado or Staked Plains down through the Texas Panhandle.

The only place between Mexico and Canada where the Tertiary sediments are preserved as they lapped up against the Front Range is at The Gangplank, which is a very narrow remnant of the Tertiary surface, 16 miles west of Cheyenne that rises gradually to the summit. Instead of flanking the Laramie Mountains as the Oregon Trail did the Union Pacific Railroad found and utilized The Gangplank to get over the mountains. (Were is not for The Gangplank, the settlement of the West would have been delayed.) The Gangplank is also the route of the Lincoln Highway, now I-80, and at one point there is barely enough width for the highway and the railroad.

The High Plains includes the extraordinary Sand Hills of western Nebraska that cover about a quarter of the state. This is good cattle ranching country. The sand catches the precipitation and much of it seeps down to recharge the Ogallala Aquifer.

The province is a low rainfall natural grazing area, but many of the ranches were subdivided and put to the plough from around 1910 to 1920. The inevitable happened in the 1930s when the area became known as the "Dust Bowl." Now when you fly over the area in parts of Nebraska and Kansas, you may be surprised to see a very large number of green circles on the ground. These circles, most of which are a half-mile in diameter, are irrigated with central pivot sprinkler irrigation systems. You will see many of these circles if you travel on I-70 through eastern Colorado.

ZOOM IN To see an air photo showing the Great Plains sprinkler irrigation circles, go to Aerial Photo. This is around Burlington, CO, whose longitude is West 102 degrees, whose elevation of about 4,200 feet, and whose annual precipitation is about 17 inches.

This irrigated area of the High Plains accurately discloses the outlines of the underlying Ogallala Aquifer in the sand and gravel of the Ogallala Formation underlying western Nebraska and continuing south under Kansas and the Panhandles of Oklahoma and Texas. The water is being pumped out faster than it is being replenished, and water levels have declined by up to 200 feet in Texas and up to 100 feet in the central High Plains. Furthermore, when the water is withdrawn, the aquifer may collapse, and the pore space can not be restored. So the irrigators are robbing the bank of water needed for future domestic use.

The Scotts Bluff National Monument is located in the North Platte River Valley. It has an Oregon Trail museum, and it has preserved wagon ruts of the trail. Near Bayard, which is 23 miles east of Scotts Bluff, is Chimney Rock. It is the most famous landmark on the Oregon Trail because emigrants could see it for days as they approached it. (It is 325 feet tall including the 120-foot "chimney.") For a map of the Oregon Trail go to The Oregon Trail .

Lying between the High Plains and the Rocky Mountains is the Colorado Piedmont section with Denver near the western boundary. It is surprising to find that this section has a lower elevation than the High Plains to the east. The reason is that the mantle of alluvium has been eroded away by the waters of the Platte River system. The similar Raton section lies south of the Colorado Piedmont. It is known for its high mesas and plateaus capped with lava flows. (A piedmont is a landform lying at the base of a mountain.)

The Edwards Plateau lies at the south end of the Great Plains in the heart of West Texas and is cattle, sheep, and angora goat country. It is bordered on the south and east by the Balcones Fault zone (marked by the Balcones Escarpment) that runs north by San Antonio, Austin, and Waco and between Fort Worth and Dallas and is the western boundary of the Gulf Coastal Plain. The Plateau is underlain by a thick layer of limestone. Water permeates the limestone and is discharged along the Balcones Escarpment and supplies cities located near it.

ZOOM IN To see how San Antonio is snuggled up against the Balcones Escarpment, go to Topo Map. Most of their water is pumped from the Edwards Aquifer, which is within the Balcones fault zone.

In the Plains Border section between the High Plains and the Central Lowlands the boundary between the Great Plains and the Central Lowlands is difficult to establish because this is a transitional section. It runs from southern Nebraska down across Kansas and western Oklahoma.

Other physiographic sections of the Great Plains include the Pecos section that is a long trough between the High Plains and the Basin and Range province on the west and the Central Texas section lying north and east of Edwards Plateau.
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ROCKY MOUNTAIN SYSTEM

To get a feel for the Rocky Mountain topography, go to the Aerial View. For an index map that labels what you saw on the aerial view go to Index Map.

The present reincarnation of the Rockies started forming about 70 MYA to 55 MYA by what is called the Laramide Orogeny. In places east/west compression caused great slabs of rock to be pushed over the adjoining surface. In the eastern part of the province crystalline rock pushed up and shoved aside overlying slabs of sedimentary rocks. (The eroded exposed ends of some of the slabs are now hogback ridges paralleling the mountains.) About 52 MYA countless great fissures opened up in northwest Wyoming and poured out lava. About 30 MYA The Rockies were buried again with sediments including volcanic ash and sand. The region was quite flat with only a few peaks sticking up through the sediments. Rivers meandered around without regard to buried mountains. Then about 10 MYA the region began to be uplifted and rose about a mile in elevation. Erosion accelerated and the period called the Exhumation of the Rockies began. The meandering rivers cut down and across mountains in a haphazard fashion.

The problem in this incredible story of mountains rising, thrusting, and getting buried and exhumed is that it is difficult to explain in terms of plate tectonics. The mountains are a long way inland from the active plate boundary to the west. Theories are plentiful, but the problem is still being sorted out.

Northern Rockies

There are three major and one minor mountain groups in the Northern Rockies. First, there are the folded and faulted sedimentary mountains in the Idaho Panhandle north of Lake Pend Oreille and in the northwestern corner of Montana. (Not generally included in the Rockies is the Selkirk range to the west along the Washington/Idaho border.) The ranges of this group are separated by north-south valleys, the largest of which are the Purcell Trench and the Rocky Mountain Trench in which are the headwaters of the Columbia River, which flows north in the trench before heading west. The Canadian Rockies lie to the east of the trench. Following along the Continental Divide east of Kalispell is the Lewis Overthrust, which is a textbook example of overthrusting. Here a 300-mile swath of the crust has been thrust over adjacent younger crust to the east a distance of 50 miles! Glacier Park in The Waterton/Glacier International Peace Park is in this overthrust area. For the full story of Glacier Park geology go to Glacier National Park Geology.

To the south is a second group including the Coeur d'Alene, Clearwater, Sawtooth, and Salmon groups of mountains developed on the Idaho Batholith, which is a big granitic intrusion. It is interesting to note that this intrusion occurred in late Jurassic and Cretaceous times at the same time the Sierras were being intruded. The Bitterroot Range lies along the eastern edge. I knew the Bitterroot Valley when it was unspoiled by development, but I understand that now it has been gentrified. Lolo is a small town just south of Missoula in the Bitterroot Valley where Highway 12 goes west over the Bitterroot Mountains. This mountain pass was taken by Lewis and Clark. Later, we will return to Missoula and tell about the catastrophic Missoula floods.

The third group of mountains and valleys are in southwestern Montana and are fault-block mountains similar to those in the Basin and Range region. At the eastern edge of the Northern Rockies south of Great Falls is a fourth and minor group of mountains consisting of the Big Belt, Little Belt, Castle, and Crazy Mountains.

Middle Rockies

The Middle Rockies are bordered on the west by the Columbia Plateau and the Basin and Range province, on the north and northeast by the Great Plains, on the east by the Great Plains and the Wyoming Basin, and on the south by the Colorado Plateaus. The Bighorn Basin lies in the northeastern part and is bordered on the east by the Bighorn Mountains, on the southwest by the Bridger, Owl Creek, and Shoshone Mountains, and on the west by the Absaroka Mountains. The Basin opens to the north. The Bighorn River crosses the Bighorn Basin, and instead of flowing north through the open end of the Basin, it cuts smack across the Bighorn Mountains. (Recall the burial and exhumation of the Rockies.) The Bighorn River separates the Shoshone Range from the Wind River Mountains, which is separated from the Wyoming Range by the Green River. The Wasatch Mountains run north and east of Salt Lake City where they intersect the Uinta Mountains that perversely run east and west "across the grain" to the southwest of the Wyoming Basin.

A gem of the Middle Rockies is the Teton Range south of Yellowstone. The Teton Range is a tilted fault block like the Sierras. It is hinged on the west along the Idaho/Wyoming line and tilted up on the east adjacent to Jackson Hole, which is also a tilt block that is hinged on the east and dropped down on the west facing the Tetons. The neighboring Rocky Mountains are about 50 MY old, but the Tetons are only about 10 MY old. An exquisite scene is the view across the Snake River Valley in Jackson Hole of the rugged face of the Tetons.

ZOOM IN To look at the neighborhood of the highest peak, Grand Teton, go to Topo Map. Notice Cascade Canyon, which was the path of glaciers that bulldozed up a terminal moraine along the east side of Jenny Lake. About 4 miles southwest of Jenny Lake, which has an elevation of about 6,800 feet, you will see Grand Teton peak, which rises to 13,770 feet. Notice Teton Glacier on Grand Teton's north shoulder. If you decrease the resolution a couple notches, you will see that the easternmost Tetons line up along the Teton fault line like an NFL team lines up at the line of scrimmage. To learn more about the geology of the Tetons go to Grand Teton, Journey Through the Past.

Yellowstone Plateau, home of Yellowstone Park, is a section of the Middle Rockies. Since it is a plateau rather than a mountain and since its volcanism is related to the Snake River Plain in the Columbia Plateau, that is where I discuss it.

Geologically, the Middle Rockies are mainly folded mountains like the Bighorn Mountains, which have been eroded exposing the crystalline rock core (mainly granite), uplifted fault blocks like the Tetons, and the remains of volcanic plateaus like the Absaroka Mountains.

Wyoming Basin

The Wyoming Basin, which is actually three basins drained by three rivers, connects with the Great Plains on the northeast and has a similar plains type topography. It separates the Southern Rockies from the Middle Rockies. For an interesting account of what happened geologically in the Wyoming Rockies go to Geologic History.

The Wyoming Basin led settlers to the famous South Pass over the Southern Rockies on the Oregon Trail. The emigrants followed the North Platte River from Scottsbluff (Nebraska) to Fort Laramie (Wyoming), did an end run around the north end of the Laramie Mountains, passed the present site of Casper, went southwest along the Platte to Independence Rock, and then followed the Sweetwater River past Devil's Gate to South Pass, which was a grassy crossing of the continental divide. The Wind River Mountains are to the north. The emigrants left the Sweetwater and followed the Little Sandy and Big Sandy Rivers to the Green River, which many crossed by ferry. They took the Bear River past Soda Springs. (Highway 30 follows the route.) After leaving the Bear River, the trail forked with one trail going south to California and the other north to Oregon. The next points north were Shoshone Creek, Ross Creek, Port Neuf River near the present site of Pocatello, and finally the Snake River, which they followed to the Oregon Territory.

For a great diary of an emigrant go to Diary of Mrs. Amelia Stewart Knight, 1853. In 1860 my maternal great grandfather brought his family also with eight children over the Oregon Trail, including my grandfather, who was then a teenager. As in the case of the Knight family, a baby was born on the way, but it died. It was buried in the wagon tracks to reduce the risk of being dug up by animals. My paternal grandfather had the good sense to wait and take the train in 1875.

For a good Web site telling about another famous trail go to Santa Fe Trail. For a good Web page on the Chisholm and other cattle trails go to Along the Chisholm Trail.

Southern Rockies

The Southern Rockies are bordered on the north by the Wyoming Basin, on the east by the Great Plains, on the south by the Basin and Range province, and on the west by the Colorado Plateaus. A good place to start in the Southern Rockies is Rocky Mountain Geology, Rocky Mountain National Park. For more detail over a broader area see Rocky Mountain Geology South-Central Colorado. Also see Garden of the Gods.

The Laramie Range is the "forefinger" sticking up to the north around which the North Platte River flows out of the Wyoming Basin. The eastern belt of mountains continues south with the Colorado Front Range, which ends a little south of Pikes Peak, the Wet Mountains, and the Sangre de Cristo Range, where four tributaries of the Arkansas River head up. The western belt of mountains include the Park, Gore, and Sawatch Ranges and the Elk Mountains and the San Juan Mountains reaching down into New Mexico west of the Rio Grande River. US Highway 70 picks its way west from Denver through the Front and Gore Ranges. Four basins separate the two mountain belts: North Park (the head waters of the Colorado River), Middle Park, South Park, and the San Luis Valley.

ZOOM IN To take a look at the Great Plains/Southern Rockies boundary at Denver, go to Topo Map.

The geologic story is that the mountain cores of metamorphic and granitic rock pushed up through the overlying sedimentary rock. Great slabs of this rock are stacked on the slopes of the mountains forming hogbacks such as the Dakota Sandstone hogbacks along the eastern slop of the Colorado Front Range.

Back to the Beginning

COLORADO PLATEAU

The Colorado Plateau province is actually four plateaus, and its world-class attraction is the Grand Canyon. To get the lay of the land, go to the Aerial View and to the Index Map.

This province is an incredibly stable island of flat-lying sedimentary rock surrounded by chaos. (It belongs over in the stable Great Plains province instead of in this rowdy neighborhood.) It is bordered on the east with the Rockies in which the rock strata are compressed, folded, and faulted, and on the west with the Basin and Range province in which the crust is stretched and faulted, and the blocks are tilted.

Another extraordinary thing about the Colorado Plateau is how it has been uplifted. The Kaibab Uplift has been elevated to about 9,000 feet above sea level. When you stand on the rim of the Grand Canyon in some places and look down, the Colorado River is a mile below you. When you fly over, you may be stunned by how much rock the river has excavated. In places you see the meanders made by a once slow-moving river, but now they are deeply incised. All of these are signs that the plateau was uplifted, but there is considerable speculation about the reason for it.

If you want to read the geological history of the Colorado Plateau Region, go to the bottom of the Grand Canyon in Granite Gorge and see the 1.7 billion year old continental Precambrian basement rocks. Then climb up the mile high wall through about 40 strata or formations through hundreds of millions of years of deposition to the top to the Kaibab Formation that was deposited about 250 MYA at the end of Paleozoic time. The strata are like pages of a book recording the geological history. Some pages are missing either because the land was above water or the sediments were eroded away. In fact, a whole chapter is missing that is called the Great Unconformity, which is a gap in the rock record of over 1 billion years.

ZOOM IN To see a topo map covering the area around Grand Canyon Village, which is about 75 miles northwest of Flagstaff, AR, go to Topo Map. The contour interval is 50 meters. The Colorado River elevation at this point is about 2,500 feet. The elevation at the overlook is about 7,400 feet. (Not a good place for an acrophobe!) The canyon is a minimum of about 10 miles wide at this point.

For more excellent information go to Grand Canyon Explorer. The Grand Staircase is a sequence of strata from the Grand Canyon to Bryce Canyon in Utah. See Grand Staircase.
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BASIN AND RANGE

For a USGS description and maps of this incomparable province go to Basin and Range Province.

This province consists of five sections. The Great Basin Section lies northeast of the Colorado Plateau and consists of about 100 separate basins across Nevada and western Utah. To appreciate the extraordinary topography of this province go to an Aerial View of Basin and Range Province and to the Basin and Range Index Map.

As you look down on this unique Basin and Range landscape you might get the same impression that an early observer got who said that the mountains looked like an army of caterpillars crawling northward out of Mexico. The landscape consists of long narrow tilt fault block mountains that run parallel north and south with desert basins or valleys in between. The mountains average about 10 miles in width and are typically about 50 to 75 miles in length. The Great Basin has no outlet to the ocean.

ZOOM IN For a closer look at the "caterpillar" topography of the ranges and basins go to the Topo Map. These impressive mountains that have elevations of from 9,000 to 12,000 feet are in the Toiyabe National Forest in the center of the Great Basin south of Highway 50. The basins have elevations of 5,000 to 6,000 feet and have no drainage outlet to the ocean.

What could cause such a strange arrangement of mountains and basins? Picture a dozen books between two bookends. Pull some books up (the fault block mountains) and push adjacent books down (the downdropped basins) One bookend is the Sierra Nevada fault block range that tilted to the west and has a steep fault scarp facing east. The other bookend at the eastern edge of the Great Basin is the Wasatch fault block range with a fault scarp facing west. Whereas the Rockies resulted from compression and uplifting, the Great Basin was stretched east and west by about 100%, and it is still being stretched! The cause of the incredible stretching is a subject of considerable speculation, but we will see later how it is related to the Pacific Plate motion and the "swinging door" effect. Faulting is still common in this region, and earthquakes are not uncommon. A mountain goes up or a basin drops leaving a fault scarp as evidence.

One notable feature in the Great Basin is the Great Salt Lake in Utah, which is a puddle compared to the Lake Bonneville that was here 15,000 years ago. It spilled over its rim at Red Rock Pass and slowly eroded down to a soft rock formation. Then the escaping water swiftly cut through 300 feet and released the top third of the water in this huge lake. The water flowed into the tributaries of the Snake River in a deluge of about 300 times the normal maximum flow of the Snake and spread over parts of the Snake River Plain leaving behind its load of rocks and boulders. Boulders were rounded from their pummeling in the raging waters. When I first traveled from Boise to Twin Falls, I saw a large flat area where many of these boulders were lying. There was a discount gasoline dealer in Boise by the name of The Stinker, who had lots of roadside signs. At this site the sign said, "Petrified watermelons. Take one home to your mother-in-law." For more information go to The Lake Bonneville Flood.

Another notable feature of the Great Basin on the southwestern edge in California is Death Valley. It has the distinction of being the hottest, driest, and lowest point (282 feet below sea level) in the United States. Surprisingly, only about 85 miles to the west is towering Mt. Whitney in the Sierra Nevada that is the highest point in the lower 48 states, 14,495 feet. To take a fascinating virtual geology tour go to Death Valley Geology Field Trip.

The agriculture of the province is mainly ranching, but the Imperial Valley, which lies at the south end of the Salton Trough along the Mexican border west of Yuma and is mostly below sea level, is very productive due to irrigation from the Colorado River. You may have noticed that the first of the season strawberries, cantaloupes, etc. come from there. Prior to 1905 there was no permanent lake in the Salton Basin even though it was flooded intermittently by the Colorado River, but in 1905 engineers tried to increase the flow of irrigation water with a cut in the bank of the Colorado River. However, flood waters broke through a canal bank and much of the river poured into the Basin creating the Salton Sea. It hasn't dried up again because of being fed by irrigation drainage water from the Imperial Valley and Coachella Valley. The Salton Sea is a recreation and wildlife haven, but it may be headed for an environmental collapse due to agricultural and other pollution and salinity. There have been several major bird die-offs.

Geology of Nevada has detailed information on the area.
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COLUMBIA PLATEAU

To call the Columbia Plateau a "plateau" is deceiving because the term usually means an extensive upland region in relation to its surroundings. The only times I ever have the feeling that it is a plateau is when I drive from the Columbia River up out of its gorge on either side. It is actually more of a basin than a plateau. For an introduction go to Columbia Plateau Province and to Subprovince Index Map. Later you may want to explore some of the links at Columbia Plateau.

Snake River Plain

Yellowstone Park sits on a "hotspot" or mantle plume at the east end of the Snake River Plain. The best example of a hotspot is found in Hawaii. Go to Maps and Images of Hawaii and look at the Hawaiian Islands and the Emperor Chain. This chain is explained by a mantle thermal plume rising to the surface and producing volcanoes. Volcanism is presently active on the big Island of Hawaii, which is the southern most island of the chain. Picture a hotspot in the mantle that is more or less fixed in position. Now picture the tectonic plate on which the island chain is riding moving across the hotspot over time. The relative position of the islands shows the direction and amount of movement that the plate has undergone. In other words, the island chain is an accurate physical historical record of plate movement thanks to the hotspot. For further reading on hotspots go to Hotspots: Mantle Thermal Plumes.

About 17 MYA volcanoes started at about the Oregon line and left a trail across the Snake River Plain. The hotspot didn't move, but the North American plate did. Investigators have found evidence that at least 142 volcanoes clustering in about seven volcanic fields have resulted from this hotspot. The hotspot now resides under the Yellowstone National Park, and it isn't difficult to be convinced of its presence when you see all of the geysers, hot pools of water, and bubbling mud pots.

I wonder how many vacationers that go to the park know that most of it sits on top of the mother of all volcanoes and that it is still active. Over the last 2 MY volcanoes have built up the Yellowstone Plateau. When the Yellowstone volcano last exploded, it left a caldera or basin that is about 40 miles in diameter. It has had three major eruptions, and the good news is that they were about 600,000 years apart. The bad news is that the last one was about 600,000 years ago. (Don't let that change your plans for a trip to Yellowstone. It could be tens of thousand years before it blows again. In any case it will eventually blow again.) The largest of these volcanoes ejected about 2,000 cubic miles of lava and ash. This is over 5,000 times the amount ejected by Mt. St. Helens. Yellowstone is an appropriate name for the park because there is a lot of yellowish volcanic rhyolite rock in the area. For more on the exciting Yellowstone story go to Volcanic History of the Yellowstone Plateau Volcanic Field and to Volcanism.

ZOOM IN To see the Snake River Plain in the Twin Falls area go to Topo Map. This is an irrigated area that produces lots of Idaho potatoes. Notice the blue irrigation canals.

Columbia Basin

From the Snake River Plain we go northwest into Oregon and Washington and into the Columbia Basin. (I was reared in the Pullman area near the Idaho line.) Let's turn back the geological clock to late Tertiary time about 17 MYA. When you think of a volcano, do you think of a cone-shaped stratovolcano like Mt. Fuji? A much different kind called a basalt flood volcano visited this area over 300 times over the next 11 million years. It piled up layer upon layer of basalt to a depth of over 2 miles. Long fissures in the earth opened up in Southeastern Washington and Northeastern Oregon, some may have been a hundred miles long. When a fissure opened, we can say without too much hyperbole that "all hell broke loose." Lava poured out in a molten sheet that spread across the region like a prairie fire and incinerated everything that could burn. When you drive through the Columbia River Gorge, you can see layer after layer of basalt rock consisting of five or six-sided columns, and each layer is capped with a thin charred layer of ancient soil. All of our better country roads were graveled with crushed basalt. I was in high school before I discovered that not all rock is basalt.

For more on the basalt flooding go to Columbia River Flood Basalt Province and to . Also see The Geology of Washington. This latter page also notes the origin of the rolling Palouse Hills in Eastern Washington where I was reared. These hills are large loess (mostly silt) dunes. Their origin is not clear-cut. Most of the loess probably blew in from a desert area to the southwest. Some came from glacial outwash silt, and some came from volcanic ash.

Southwest of Spokane there is an area called the channeled scablands. Early in the 20th Century there was a lot of debate about what caused these scabland features. In 1910 geologist J. Harlen Bretz theorized that the cause of these features was a catastrophic flood. This was at a time when geologists did not cotton to catastrophism. One problem was to explain where all of the water could come from.

ZOOM IN To see what the channeled scablands look like, go to Aerial Photo. This is near Sprague, WA. The flood ripped out many holes in the basalt, and they filled with water and have been stocked with fish, which I have pursued on a few occasions.

The stage was set for the flood when the last of the continental glaciers visited Northern Washington and Idaho. A lobe of the glacier dammed the Clark Forks River, and water backed up forming the enormous Glacial Lake Missoula in western Montana. The pressure of the water floated the ice in the dam and released nearly 500 cubic miles of water with a flow of ten times that of all the rivers in the world. It covered the present site of Spokane with hundreds of feet of raging flood water. It divided into two major parts and raced across the Columbia Basin. One branch went over an avalanche, which today is called Dry Falls near Soap Lake and is about three times the size of Niagara Falls. The outlet through the Cascade Range for the Columbia River was the Wallula Gap near Pasco, which was way too small to take that much water. The water ponded up to form temporary Lake Lewis. This flooding episode happened not once but over forty times! The land that it scoured is now appropriately called the channeled scablands. It wasn't until the 1960s that Bretz's explanation was accepted, but now the floods are often called the Bretz Floods. For the full story and for maps go to The Great Floods of Glacial Lake Missoula with five Web pages.
Back to the Beginning

PACIFIC MOUNTAIN SYSTEM

To prepare for our tour, go to Pacific Province. To get your bearings and see the provinces of the region go to Province and Subprovince Boundaries.

Cascades-Sierra Mountains

This venerable string of mountains is a 1,000-mile-long wall that puts a lot of land to the east in a rain shadow.

Cascades

From the Columbia Basin we drive on Highway 20 across the North Cascades, which is a spectacular drive. These mountains started as a docking area for "immigrant" terranes consisting of island arcs, minicontinents, ocean floor, etc. that were brought in on ocean plates and shoved off on the edge of the continent. They were then uplifted, eroded, and invaded with volcanoes and covered with ash and lava and further eroded with water and ice. For more details and pictures go to North Cascades Geology for the fascinating story of their origin.

ZOOM IN To see the topo map of one of the premier Cascade peaks, Mt. Hood east of Portland, go to Topo Map. You can see the Columbia River Gorge and the town of Hood River to the north.

The great volcanic mountains are the backbone of the Cascade Range. Washington has five stratovolcanoes that built up steep cones, Oregon has six, and California has two in the Cascade chain. These stratovolcanoes are caused by oceanic plates subducting under the continent. Since the rising magma under the Cascades is the sticky kind, the volcanoes are explosive and dangerous.

When Mt. St. Helens literally blew her top in 1980, volcanic ash drifted and covered much of Eastern Washington. My mother lived in Moses Lake, which is about 160 miles northeast of the mountain. She could hardly believe what was happening on a bright Sunday morning as the sky darkened, and a couple of inches of volcanic "ash" were deposited on her lawn and roof. Stratovolcanoes pose other more frightening dangers. The north face of Mt. St. Helens suddenly turned into an avalanche that completely buried the gorgeous Spirit Lake, ash blasts and raging mudflows raced down the mountain, and a searing blast of wind leveled over 600 square kilometers of forest. (Mt. Vesuvius released hot poisonous gasses that killed the people of Pompei, and then it unceremoniously buried them with ash.)

The highly populated area of Seattle/Tacoma has Mt. Ranier in its back yard, which is a scenic wonder but an ominous threat. If that were not enough, Seattle also has a serious earthquake threat. Portland has the beautiful but dangerous Mt. Hood nearby. See Mt. Hood: History and Hazards.

Serious volcanism began in the Cascades in the Early Pliocene Epoc (only about 4 MYA) and has continued to the present. To get more information go to America's Volcanic Past - Cascade Range. To get the pictures and stories of the main peaks, go to Cascade Range Volcanoes Summary.

Sierra Nevada

In Spanish "nevada" means snowfall or a snow-capped mountain. For excellent summaries of the geologic history of California and the Sierras go to A Brief Geologic History of California. The events include the rafting in on an oceanic plate "immigrant" terranes that were jammed against the continent, sedimentary rock formation some of which was eventually metamorphosed, the subduction of the Kula and Farralon plates causing widespread volcanism and plutonic action, which emplaced the great granitic Sierra batholith, faulting, and uplift. The largest uplift (more to the south than the north) came about 5 million years ago. The Sierra Nevada was uplifted as one tremendous fault block about 400 miles long and some 70 miles wide and tilted up on the east side like a great long barge that has risen on a swell of water and is listing to the west.

ZOOM IN To see the scarp along the eastern side of the Sierras south of Susanville, go to Topo Map. On the steepest slope the elevation drops almost a thousand feet in a third of a mile. Notice the relative flatness on top. Honey Lake, which is a large shallow desert lake in the Basin and Range Province, is on the northeast.

By the early Miocene the Farallon plate was consumed, and by late Oligocene the Pacific Plate touched the continent. Since the Pacific Plate moves northwesterly, subduction stopped and was replaced by the fearsome San Andreas Fault. In the middle of the Miocene Period California appears to have swung westward like a swinging door stretching the Basin and Range Province increasing the distance from a future San Francisco to a future Salt Lake City by a couple hundred miles.

The crest of the Sierras rises over 9,000 feet. (The highest peak, Mt. Whitney, rises to 14,495 feet.) This high barrier catches precipitation in the form of snow from winter storms and puts the Basin and Range province in a "rain shadow." The streams flowing west and the glaciers that followed them have carved out deep canyons including Tuolumme Canyon, Yosemite Valley, and Kings Canyon. Much of the granitic rock is exposed in these canyons, as a visit to Yosemite Valley will reveal. Here is another quick synopsis of the history of the Sierras in general and the Yosemite Park area in particular: Yosemite Geology.

The foothills along the western edge are famous for their gold treasures. Gold bearing quartz veins formed in the foothill zone. Water percolated to great depths and was superheated dissolving gold and quartz, and that requires really hot water under really high pressure. The solution rose and penetrated cracks and deposited gold and quartz veins. Uplift followed by erosion exposed gold veins and deposited the gold in streambeds as "placer gold." In mining the gold many of the areas with placer deposits were seriously eroded by hydraulic mining during the Gold Rush period.

Pacific Border

The Pacific Border mountains starting at the north end include the Olympics, Oregon Coast Range, Klamath Mountains, California Coast Range, and the Los Angeles Range. These mountains are the result of ocean plates plunging under the continent and ocean floor crust and sedimentary rocks getting scraped off and bulldozed against the continent. In places magma intruded the mountains and was extruded forming volcanic landscapes.

Olympic Mountains

Once I drove around the Olympic Peninsula to Port Angeles on the Straits of Juan de Fuca. To kill some time before dinner, I drove up the adjacent mountain to Hurricane Ridge and looked out over the Olympic Mountains. I was blown away! These mountains impressed me as much as the Austrian Alps. Here is a picture of some of the Olympic Peaks. For more on the origin of the Olympic Mountains go to Geological Features.

Oregon Coast Ranges

The Oregon Coastal Range joins the Olympic Mountains and runs south, is severed by the Columbia River, and continues south to the Klamath Mountains. A scanty discontinuous coastal plain lies between the range and the ocean on the west. Where there is no plain, there are often impressive headlands being cut away by wave action. The Puget Trough containing the Puget Sound and the Willamette Valley borders the northern part of the range on the east.

I spent a memorable summer when I was in college working for the Soil Survey, which was mapping soils south of Olympia. I was astonished at the amount of rounded gravel underlying much of the area. It was part of the glacial outwash from the melting Puget Lobe of the continental glacier that started pushing down from Canada about 18,000 years ago and sculpted the Puget Sound. It stopped just south of Olympia.

The Willamette Valley drained by the Willamette River extends south of Portland taking in Salem, Corvalis, and Eugene. I will mention two events that left their mark on the valley. The first was the aforementioned flood basalts from the Columbia Plateau that came through the Columbia Gorge and flowed down the Willamette Valley. A second event was the Lake Missoula flood that backed up into the Willamette Valley when it came through the Columbia Gorge. Since then the valley has been filled with river alluvium.

The sprawling Klamath Mountains are shoehorned in between the Oregon and the California Coast range and show more geological kinship to the Sierra Nevada than to the Coast Ranges. One of the attractions is the groves of coastal redwoods on the south. The Rogue and Klamath Rivers cut across the mountains. Once I wanted to cross from Highway I-5 to 101 and naively took what turned out to be a white-knuckle road (whose status was grossly misrepresented on our map) along the Klamath River. It was primarily a logging road with alarming skid marks near each curve with the added hazard of no guardrails to prevent one from going over the cliff into Klamath river far below.

California Coast Range

The California Coast Range is about 400 miles long and averages about 50 miles wide including its ranges and valleys. North of San Francisco the valleys include the Sonoma and Napa Valleys, which are well-known to wine lovers. Across the Golden Gate Bridge I often go to the Marin Headlands overlooking the Golden Gate. Road cuts reveal the Franciscan Complex of rocks that record their tortured past. Thin twisted strata of chert are often vertical as a result of the bull-dozing effect of the ocean plate ramming under the continent. Chert is a lithified deep ocean silica sediment consisting of interlocking crystals of quartz. Part of the silica was derived from the silica shells of dead single-celled organisms.

One of my favorite places along the coast is the Point Reyes National Seashore Park north of San Francisco. When you drive west from Point Reyes Station across the Olema Valley, you go from the North American Plate to the Pacific Plate as you cross the San Andreas fault zone a few tens of yards wide. (After the 1906 earthquake fences crossing the fault were offset as much as 20 feet. For a picture of the rip go to The Geology of Point Reyes.) The Pacific Plate is sliding past the North American Plate about two inches per year averaged over a long period. At some point in time the Point Reyes Peninsula will be off the coast of Alaska and so will Los Angeles because it is farther south on the same plate.

ZOOM IN To take a closer look at Point Reyes and the route of the San Andreas fault, go to Topo Map. The fault runs down Tomales Bay and Olema Valley, goes straight into the ocean, and then comes ashore just south of San Francisco and runs down the Peninsula where its route is marked by the Crystal Reservoir. You can see that Point Reyes appears to be "temporarily moored" on the coast of Marin County.

For more information on the Coast Range and the Transverse Ranges of Southern California go to California's Coastal Mountains.

Transverse Ranges

At the south end of the Great Central Valley we go over the Tehachapi Mountains, which seem to be the westward turned tip of the Sierra Nevada but have a different history. As we enter Southern California, the San Andreas Fault jogs east. Also there are several small mountain ranges that go east and west instead of north and south like the rest of the coastal ranges, so they are called the Transverse Ranges. About 5 MYA the East Pacific Rise spreading center opened up the Gulf of California and compressed the crust to the north, which raised the Transverse Range, and they are still rising. This region is dominated by the Los Angeles Basin, which is filled with up to 6 miles of sediments and didn't surface above the water until about 100,000 years ago.
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California Trough (Great Central Valley)

The California trough is about 400 miles long and about 50 miles wide reaching from the Klamath Mountains on the north to the Tehachapi Mountains on the south. It is a great structural trough or basin that has been filled with marine sediment and alluvium from the Sierras and the Coast Range. The valley floor is nearly flat. The fertile soil, availability of irrigation water, and favorable climate make the valley a world-class agricultural area.

The Sacramento Valley in the northern part of the trough is drained by the Sacramento River, which terminates in the "Delta" (marshy lowland at the confluence of the Sacramento and San Joaquin Rivers). Much of the Delta has been diked and drained and used for crops. Many of the dikes are old and poorly engineered. In June 2004 a levee failed and 11,000 acres of land in crops were flooded causing about $75 million in damage. The Sacramento Valley produces a staggering variety of crops are grown including rice, safflower, peaches, almonds, kiwis, and tomatoes.

ZOOM IN A curious anomaly sticking up in the middle of the broad Sacramento Valley northwest of Yuba City is the Sutter Buttes. They are an igneous island in a sea of deep sedimentary deposits. They rise about two-thousand feet above the valley floor from a very circular volcanic lava dome about 10 miles in diameter. Go to Aerial Photo.

The San Joaquin Valley is the southern part of the trough, and the northern part is drained by the San Joaquin River, which empties into the Delta. The river receives water from the Sierra runoff, but not enough to supply its heavy agricultural demand. A lot of water is imported from the Sacramento River system. The San Joaquin Valley has 5 million acres of irrigated land that produce a great variety of fruits and nuts and a large amount of irrigated cotton. The south half of the San Joaquin Valley is the Tulare Basin that drains into Tulare Lake, which has no outlet. The Lake dried up in the drought of the early 1930s, and much of it was farmed. However, it formed again in the 1997 flood.
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INTERIOR HIGHLANDS

Ozark Plateaus

Now we take a big hop east to the "Ozarks," which are mostly in southern Missouri and northern Arkansas. Ozarks is a vague term, but some geographers include in this region not only the stream-dissected Ozark Plateaus, which is a dome-shaped uplift consisting of the Springfield and Salem Plateaus, but also the Boston "Mountains," which rim the plateaus on the south. There is a lot of limestone or dolomite in the plateaus, which accounts for the caves.

The St. Francois Mountains are quite conspicuous in the northeastern section west of Highway 67 in the Rock Pile Mountain Wilderness. Here you see the oldest rock in the province, which is Precambrian granite. Black Mountain is in the center of the section, and its elevation is about 1,300 feet.

The music town of Branson, Missouri, is in the middle of the Ozarks. The surrounding landscape is a lot of ridges and hollows. The ridges all have about the same elevation, i.e., 900 to 1,200 feet indicating that area is an eroded plateau. The relief may be from 100 to 300 feet depending upon how far up the hollows you go. If you go south across the Buffalo River and into the Boston Mountains, the elevations rise up to about 1,600 feet.

ZOOM IN To see the Ozark topography in the Branson Missouri area, go to Topo Map. Switch to the aerial photo and see that many of the hilltops have been cleared for cultivation.

Ouachita Mountains

The Ouachita Mountains are separated from the Boston Mountains by the Arkansas River Valley. Little Rock is located on the eastern edge of the Ouachita Mountains section on the Arkansas River not far from its junction with the Mississippi, but the more impressive mountains are west of Mena, AR, which is situated in the center of the section. These mountains have parallel ridges running east and west and have elevations up to about 2,000 feet. They are steeply folded and have sandstone ridges that show their geological relationship to the Southern Appalachians. The Beech Creek National Scenic Area is in the province.

ZOOM IN To get a feel for the Ouachita Mountain topography, go to Topo Map.

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ATLANTIC PLAIN

In the USGS classification the "Atlantic Plain" division consists of the Coastal Plain province above water and the Continental Shelf province below the water line on the coast. To see an aerial map showing the entire Coastal Plain, go to Aerial Map. Coastal Plain sediments eroded from the Appalachians were deposited in the Atlantic and in the Gulf to build up the Continental Shelf and Coastal Plain and became sedimentary rock. To see a generalized cross-section of the Atlantic Coastal Plain, go to Coastal Plain Geologic Cross-section.

Along the Atlantic seaboard the western margin of the Coastal Plain is the Fall Line separating it from the Piedmont Plateau. It is called the Fall Line because there are falls or rapids all along it because the hard metamorphic and igneous rocks underlying the Piedmont Plateau resisted erosion. In Colonial days the Fall Line was a good place to site water-powered mills, and it was also the western terminal for river boat transportation. Cities that were founded at the Fall Line include Richmond, Raleigh, Columbia, and Augusta.

ZOOM IN Richmond straddles the Fall Line and is the only urban center that has white water rafting in the city. To see the rapids go to Aerial Photo showing the Richmond Rapids.

The Coastal Plain includes five sections. The Embayed section runs from New Jersey through the north half of the North Carolina coast. During the periods of continental glaciers the sea level was much lower, and the Continental Shelf was exposed and was eroded by rivers. When the sea level rose, these river valleys were "drowned." This is the origin of the Chesapeake Bay. For more information go to Atlantic Plain Province.

The next section on south is the Sea Island section reaching down to Jacksonville, Florida. Sea Island is an international resort off the coast of Georgia. The island is about 5 miles long and up to about a half mile wide. It is one of many barrier islands along the coast of this section. These islands are wave built offshore sand bars that protect the Atlantic Intracoastal Waterway between the islands and the mainland. There are about 1,200 miles altogether of protected waterway along the east coast. (Unfortunately, we have no such protected waterway along the west coast.)

The next section south is the Floridian section. Geologically, Florida is the emerged part of the Peninsular Arch, which is underlain with Cretaceous limestone that accounts for the large number of sinkholes and most of the nearly 8,000 lakes.

ZOOM IN Look at the number of limestone solution lakes within just a part of the city of Orlando in this Aerial Photo.

Another extraordinary feature of this limestone country is the springs. The largest is Silver Springs located 6 miles northeast of Ocala. The flow from the springs is nearly 400,000 gallons per minute, which feeds a river! The springs are fed by solution channels and cracks in the limestone.

The largest lake (of a different type) is Lake Okeechobee at the north end of the Everglades, which is a vast fresh water marsh that is in danger, see Everglades National Park.

The basement rock of southern Florida is a piece of Africa that was left behind after the breakup of Pangea and the Atlantic Ocean opened up.

The next Coastal Plain section is the East Gulf Coastal Plain taking in much of Georgia, Alabama, and Mississippi. The largest part of the plain is the belted coastal plain. The sedimentary rock strata dip toward the Gulf at more of an angle than the surface. This means that these strata intersect the surface. The harder sandstones form cuestas running parallel to the Gulf and having long gentle slopes to the south toward the sea and short steep slopes to the north. In between the cuestas are belts such as the Black Belt delineated on the south by the Ripley Cuesta cutting across Alabama a little south of Montgomery and is bordered on the north by the Fall Line Hills. The "black" refers to the soil developed on limestone rather than to the people although there were many blacks on the plantations in this cotton region. The second part of this coastal plain is a narrow strip of coastal terraces adjacent to the coast and south of the Southern Pine Hills.

There are a number of National Seashores along the Atlantic and Gulf Coastal Plains. To check them out, go to National Seashores. An example is Cape Hatteras on the Outer Banks of North Carolina.

The large Mississippi Alluvial Plain section is part of this province. Which cities in the U.S. would you think are the most vulnerable to a devastating earthquake? San Francisco, Los Angeles, or Seattle perhaps? The best answer may be St. Louis and Memphis because the New Madrid Fault is half way between the two cities and because their building codes have been so lax. The mother of all earthquakes in the U.S., a triple-header, occurred on this fault during the winter of 1811-12. Damage occurred as far away as Charleston and Washington, D.C. Large areas of land sank and other areas rose. This was not a freak occurrence. This area has a history of earthquakes, and the bedrock structure is appropriate for earthquakes. To read more about it, go to The New Madrid Earthquake.

The West Gulf Coastal Plain is also belted with cuestas running parallel to the Coast. The western border of the plain is sharply defined by the Balcones Escarpment, which skirts the Edwards Plateau, and the northern border by the Ouachita Mountains. My introduction to the coastal plain was in the East Texas Timber Belt southwest of Shreveport, Louisiana, where I joined the 75th Division, which was on maneuvers. I was also rudely introduced to chiggers and ticks. The soil is a red clay that stalled everything when it rained but dried out surprisingly soon when the sun came out.
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APPALACHIAN HIGHLANDS

For a good short summary of the long and complicated story of this division go to Appalachian Highlands Province. Appalachia is a term often used for the Blue Ridge Mountains, the Ridge and Valley Province, and the Appalachian Plateaus.

Starting with the old continental core (the stable craton = shield and sedimentary platform), one terrane (piece of crust) after another "collided" with the continent and was welded to it. In the final chapter of mountain building, the Allegheny orogeny, the Piedmont and the Blue Ridge Mountains were shoved west compressing, folding, faulting, and thrusting landscapes of the Ridge and Valley province. At this time in the Paleozoic, 300 MYA, Africa had rammed up against the margin of the continent, resulting in the supercontinent of Pangea. Then in Triassic time (200 MYA) the Atlantic began to open up due to seafloor spreading. There was rifting along the margin that resulted in faulting and basaltic intrusion and extrusion. The Palisades of the Hudson River formed when magma rose up through cracks and cooled in place leaving sills of basalt that have been exposed by erosion.

For further description of the division go to Physiographic Regions of Virginia. This shows the following succession of belts in the Appalachian Highlands, which are bordered on the east by the Atlantic Coastal Plain: 1) the rolling Piedmont, 2) Blue Ridge Mountains, 3) Ridge and Valley province, and 4) Appalachian Plateau.

Piedmont

If you travel west from the Fall Line at Richmond on US 64 to Charlottesville, you will have crossed the Piedmont, which lies between the Blue Ridge Mountains and the Atlantic Coastal Plain. This province runs from southeastern Pennsylvania through Maryland, Virginia, N.C., S.C., and Georgia and juts a little into central Alabama.

ZOOM IN To see a sample of Piedmont terrain that is just west of Richmond go to Topo Map. For a look at the land use pattern switch to the aerial photo and increase the resolution a notch.

The landscapes of this province are generally undulating to gently rolling hills on a very old worn down surface of deeply weathered igneous and metamorphic bedrock. The highly weathered rocks are called "rotten rocks" or saprolite, which crumbles easily. There are a few basin areas with sedimentary and volcanic rock. There are lots of gullies to farm around. The soils of the Piedmont are generally quite erodable and not very productive except around Lancaster, Pa., where the bedrock is limestone. (In the 1930s and 40s the Soil Conservation Service planted kudzu, which is a vine, to control gullies. Now researchers are looking for bugs and fungi that will control the kudzu. It can cover and kill a tree by robbing all of the light.) Upland cotton became an important crop in the middle of the 19th century. The boll weevil and other factors eventually caused a shift to peanuts and other crops. The Piedmont has long been an important producer of flue-cured tobacco.

Much of the Piedmont is the remains of old "immigrant" terranes. Just west of Richmond is such a terrane that has rocks that are a billion years old.

Blue Ridge Mountains

This province lies in a narrow belt running northeastward from northern Georgia to mid-southern Pennsylvania between the Piedmont on the east and the Valley and Ridge province on the west. South of Roanoke, Virginia, the range widens to as much as 70 miles and the elevations rise to as much as about 6,000 feet.

The Shenandoah National Park saddles the Blue Ridge in northwestern Virginia. One of my memorable drives was down the beautiful 105-mile Skyline Drive. For more information on the park go to Shenandoah National Park. The Blue Ridge Parkway continues on south into the Great Smoky Mountains National Park lying along the Tennessee/North Carolina border. The Great Smoky Mountains are the most impressive mountains in the eastern United States.

ZOOM IN To see the Blue Ridge Mountains where Interstate 64 crosses near Waynesboro, VA, at an elevation of about 2,000 feet, go to Topo Map. The Blue Ridge Parkway goes to the south and Skyline Drive goes to the north. You can see the Shenandoah Valley to the west.

The rocks of the Blue Ridge are mostly Precambrian and include old metamorphic and sedimentary rocks predating those of the Ridge and Valley province on the west.

Ridge and Valley Province

This province runs from Central Alabama to New York. It adjoins the southern Piedmont and the Blue Ridge province on the east and Appalachian Plateaus and the Adirondacks on the West. When you fly over the province, you see the regular pattern of parallel ridges and hollows controlled by the structural folds in contrast to the Appalachian Plateaus that have a random pattern of highlands and lowlands.

The eastern part of the province is mostly the Great Valley running from the St. Lawrence Lowland to Alabama. The Great Valley includes from north to south such local valleys as the Champlain, Hudson, Cumberland, Hagerstown, Shenandoah, and Eastern Tennessee. The western part of the province is mostly sandstone ridges and limestone and shale valleys.

Go northwest from Washington, D.C., on US 70 to Hagerstown, MD, and you will be in the Great Valley locally known as the Cumberland Valley, which is on the eastern side of the province. Continue on west on US 70 and 68, and you will be seeing ridges with elevations of from about 1,200 to 1,500 feet with relief of about 200 to 500 feet. Cumberland, Maryland, is on the west border of the Ridge and Valley Province.

ZOOM IN For a close look at a community in the Ridge and Valley Province located about 5 miles northwest of Buchanan, VA, which is on I-81, and about 20 miles northwest of Roanoke, VA, go to Aerial Photo. It is on Spreading Spring Creek in a valley (to the southeast) bordered on the northwest by Timber Ridge with an elevation of about 1,500 feet and bordered on the southeast by Little Timber Ridge. The elevation in the valley is about 1,000 feet. Notice that all of the valley is cultivated.

The Ridge and Valley sediments were deposited on a previous continental shelf and eventually became sedimentary rock with hard (sandstone) and soft (shale and limestone) strata. During late Pennsylvanian time (300 MYA) the crust was compressed during the Allegheny orogeny when Pangea was being assembled. This resulted in north-south trending arch-shaped upfolds and trough-like downfolds; however, the present topography is an expression of the erosion of strong and weak strata. The resistant sandstone forms ridges or hogbacks, and the weak shales are removed to form valleys or "hollows." So for each upfold there may be four or more ridges, and there may be a resistant ridge in the center of a downfold.

If I push one end of a throw rug toward you, several upfolds and downfolds will be made. If I push the rug far enough, some of the folds rise enough to overturn. Likewise, the crust was compressed enough in some places for the folds to overturn. In other places upper strata of the crust were thrust over adjoining crust to form overthrusts.

Some have divided the province into three north to south sections: 1) The northern section is the narrow, glaciated Hudson-Champlain section, 2) The Middle section is bounded on the west by an escarpment along the eastern edge of the Appalachian Plateau called the Allegheny Front. 3) The Southern section is similar to the Middle but has fewer ridges. This province includes the Tennessee River system.

Appalachian Plateaus

This is the last province to the west in the Appalachian Highlands. It includes the Catskills in New York and the Poconos of eastern Pennsylvania and stretches south to Tuscaloosa, Alabama.

In Pennsylvania the eastern boundary of the Appalachian Plateaus is sharply delineated by the east-facing scarp, the aforementioned the Allegheny Front. If you go west from Philadelphia on the Pennsylvania Turnpike, you reach the Allegheny Front 20 miles west of Bedford. You will climb about 600 feet up the Front, and you will then be on the glaciated Allegheny Plateau of Pennsylvania and West Virginia.

The part of the Appalachian Plateaus in Kentucky, and the part that crosses diagonally to the northeast through central Tennessee is the Cumberland Plateau. The eastern scarp of the Cumberland Plateau. i.e., Cumberland Mountain, which is the southern equivalent of the Allegheny Front, was a barrier to western migration until the Cumberland Gap was discovered. Daniel Boone was commissioned in 1775 to build the Wilderness Road, which is now Highway 25E and 58. The road went through the Cumberland Gap. To read about The Cumberland Gap National Historical Park, go to Cumberland Gap. To get there from Interstate 75, take the Highway 25E exit at Corbin, Kentucky. It is near where Kentucky, Tennessee, and the western sliver of Virginia meet and is a couple of miles east of Middlesboro, Kentucky.

ZOOM IN To get a feel for the Appalachian Plateau, go to Topo Map. This shows the Independence, WV, community. Notice the strip mines and even an oil well. Switch to the aerial photo and look at Bull Hollow in the northern part, and you will not see much cleared land.

The western edge of the plateau is deeply cut with streams making a rough topography that one might not think of as a plateau, but all of the hilltops are about the same height indicating that it was once a flat plateau underlain with horizontal sedimentary strata.

The rocks of this province are sedimentary rocks of Pennsylvanian, Mississippian, and Devonian age. I was introduced to the Devonian rocks around Ithaca at the head of Lake Cayuga, which is a beautiful area to visit in the autumn and also in the winter if you like snow, lots of snow. For more on the geology go to Finger Lakes Geology.

New England

We will end our tour with a brief look at the New England province of the Appalachian Highlands. The major uplands include the White Mountains of New Hampshire whose highest point is Mt. Washington. One warm autumn afternoon my wife and I were driving through the White Mountains, and we decided on the spur of the moment to drive up to the top of Mt. Washington, which has a modest elevation of 6,288 feet. To our great surprise, at the summit we were greeted with gale winds and thick frost blanketing everything. The highest wind ever recorded by man was an astounding 231 miles per hour on top of Mt. Washington.

Other major uplands include the Green Mountains of Vermont, the Berkshire Hills in Massachusetts, and the Maine Uplands.

The rock underlying much of the province is metamorphic rock including slates, gneisses, schists, quartzite, and marble that are folded and fractured. Vermont is known for its granite often used for monuments. The White Mountains are carved from a mass of granite. The New England province was severely sculpted by the Ice Age glaciers and the evidence is everywhere.
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SUGGESTIONS FOR FURTHER READING

If you would like a great nontechnical book to hold in your hands and read about this subject, I highly recommend Natural Landscapes of the United States (1995, ISBN 0-7872-0533-8) by James A. Henry and Joann Mossa (based on the earlier edition of E. C. Pirkle and W. H. Yoho). It is available from Amazon.com through its used book dealer network for from $20 to $30 for one in good condition. Part I covers background information on rocks, landforms, climates, vegetation, and soils. Part II covers 23 geomorphic provinces of the 50 states.

Another way to go is the regional geography route, and an excellent book is Across This Land (2002) by John C. Hudson. This book discusses the geographic regions of the U.S. and Canada including the physical features, settlement history, and present land use. It is available online for about $27.

Here are some good on-line physical geography and geology resources:

• Fudamentals of Physical Geography is a good comprehensive text on physical geography. For the chapter on geology go to Chapter 10: Introduction to Geology.
• Geography of the United States includes a large section on physiography.
• Physiographic Provinces of the United States gives information on all of the physiographic provinces.
• Virtual Geology Field Guides to the USA includes 300 virtual geology field guides to interesting sites. (Some links are broken>)
• Essentials of Geology is a very detailed overview of a geology text.
• Introduction to Geology and Geophysics is a very readable on-line geology text.
• Earth Science is a good basic on-line text.
• Examples of Landforms Depicted on Topographic maps shows an extensive variety of landforms.
• Ecological Subregions of the United States is an excellent Forest Service site on ecological provinces, which includes descriptions of geomorphology, lithology, vegetation, fauna, climate, surface water, and land use that are outlined on a map at Ecoregions of the United States.

Here are some more good books on physical geography and geology:

• Annals of Another World by John McPhee who tells the story of geologists who took him on tours through their regions and told him about the geology. This is the combination of five separate books that McPhee wrote. They are Basin and Range, In Suspect Terrain, Rising from The Plain, Assembling California, and Crossing the Craton. These are great reads, but they will be appreciated even more if you first read a little about physical geography/geology.

• Essentials of Physical Geography by Gabler, Sager, and Wise gives a good general background in physical geography including climate, ecosystems, soils, and basic geology and landforms. It is quite readable and beautifully illustrated. A new hardback 7th Edition costs $97.95 at Amazon.com, but I bought a used paperback 5th Edition (1997)in good condition for only $2.25 from an Amazon.com affiliated used book dealer. (Students must buy the latest edition, so there is a very limited market for older editions even though they are not really out of date making for great bargains. Half.com by ebay is another good source of used books. I often check their dealers' prices after checking Amazon.com.)

• Reading the Earth, Landforms in the Making by Jerome Wycoff covers basic geology and gives lots of examples of landforms and is beautifully illustrated with pictures and diagrams on every page.

• The Changing Earth: Exploring Geology and Evolution, 2nd Edition (1997) by Monroe and Wicander is a good introductory textbook covering both physical geology and historical geology. It is well-illustrated with colored pictures and charts. Used copies are available for from about $5 to $10. The new 3rd edition sells new for $92.95.

• The Dynamic Earth, An Introduction to Physical Geology 4th Edition (2000) by Skinner and Porter is a good textbook for those who would like a comprehensive book on basic geology. The is beautifully illustrated and quite readable. Good used copies are available for about $10 to $15. Amazon.com's price for a new 5th Edition (2003) is $95.95.

• Evolution of the Earth, Sixth Edition, (2002) by Prothero and Dott is a good textbook on historical geography. It is wonderfully illustrated. A like-new book is available from Amazon.com used book dealers for about $11, which is a fantastic bargain. The newer 7th Edition sells for $121.45 new.

• A Field Manual for the Amateur Geologist by Alan M. Cvancara is a good inexpensive basic geology book that focuses on the nature and identification of landforms. For a very readable brief book on basic geology with lots of good illustrations I recommend The Field Guide to Geology by David Lambert and the Diagram Group.

• There are many geology books covering regions or states that are good. The "Roadside Geology" series covers several individual states. I have Roadside Geology of Northern California, which explains a lot of geology in the context of local landscapes. A good book with a lot of detail covering the Northwest is Northwest Exposure, A Geological Story of the Northwest. Another brief, very readable book covering the Northwest, particularly the State of Washington, is The Restless Northwest, A Geological Story.

• Field Guide to Landforms in the United States, 1972, by John A. Shimer, out of print, is an excellent brief description of the physiographic provinces and small scale landforms. It has the excellent Raisz maps with names of the landform. A good used copy can be purchased from Amazon.com's used book dealers for about $14.

Your feedback will be welcome. Please send an e-mail message to me, Bob Parvin.

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Copyright (c) 2004 Robert G. Parvin.