Geography and Natural Environment of the United States

The interior of the United States consists of a vast, irregular lowland stretching from the Gulf of Mexico to the Canadian border and continuing into Alaska. Geographers interested in the development of the landscape divide these broad plains and gently rolling hills into three geomorphologic regions: the Atlantic and Gulf Coastal Plains, the Inland Lowlands (which some scholars further divide into the Great Plains and Inland Plains), and the Canadian Highlands.

The Atlantic and Gulf Coastal Plains extend north along the East Coast to the southern fringes of New England. Beneath the land surface in this region are layers of rock that are not very solid, are prone to erosion, and are relatively recent. These rock layers were deposited as seawater flowed to and from the area in geologically recent times. These low plains extend below sea level to form the continental shelf, which can extend as far as 400 kilometers from the coast.

In the north, the inland lowlands are decidedly hilly compared to the coastal plain, but rarely rugged. The Inland Lowlands are shaped like a saucer with uplifted edges and are covered by a thick layer of sedimentary rock. These sedimentary rocks are generally very flat. Differences in geomorphology within the Inland Lowlands are the result of localized erosion or, in the northern part of the region, glacial till deposited during the Ice Age.

The geology of the Great Plains differs little from that of the Interior Plains. The Great Plains are mostly sedimentary rocks. In the northern Great Plains, however, the sedimentary layers have been reshaped by dome-shaped uplifts, as evident in the Black Hills of western South Dakota. The sedimentary sequence is nearly horizontal and dips gently westward to the base of the Rocky Mountains, where Denver and Colorado Springs, Colorado, are located.

Separating the Great Plains from the Inland Plains is a low escarpment eroded by the Rockies, which is located at the eastern end of the coarse sedimentary layer that covers the plains.

This feature of a large inland lowland had a major impact on the settlement and economy of the United States. Not only is there an abundance of agricultural resources that can be exploited, but the terrain allows for easy transportation across about half of the U.S. territory without much difficulty. Because of this, the Inland Lowlands and the West played an important role in the U.S. economy. The rivers of the Inland Lowlands are almost all connected to the Mississippi River and its tributaries. This allowed for easy transportation of goods to areas west of the Appalachian Mountains and created economic concentration, cementing regional cohesion.

In the northern and northeastern part of the Interior Lowlands lies the Canadian Highlands, which are covered by old, hard crystalline rock. The southern part of the continental shelf is made up of sedimentary layers that were deposited beneath the seawater that once filled the central United States. As the surface of the Canadian Plateau eroded, it gave way to a gently undulating lowland.

The topography of the Canadian Plateau was shaped by massive continental glaciers over the past million years. The continental glaciers covered most of Canada west of the Rockies and Coast Mountains and extended south to the present-day Missouri and Ohio River valleys.

The glaciers lifted huge boulders, some weighing tons, from the surface and carried them far away. The huge round boulders scattered across the Canadian prairies were moved by glaciers. Around the glaciers, melting ice formed major river systems, creating wide channels that led to the ocean.

Glacial action washed away much of the surface of the Canadian Plateau, so today the topsoil is very shallow or non-existent. The landslides also severely altered the shape of the waterways, some of which, instead of flowing to the ocean, became the region’s lakes and swamps. For example, the “Land of 10,000 Lakes” in central and northern Minnesota is part of the southern part of the glaciated Canadian Plateau. The states of Minnesota, Michigan, and Wisconsin are located on the glaciated Canadian Plateau.

In the south, the glaciers were relatively thin and unaffected, and the high-altitude terrain altered their flow. In central New York State, for example, the glaciers were blocked from advancing by the high ground south of the Mohawk River. But in the valleys where the Mohawk River tributaries flowed, the glaciers continued to flow in small increments, making the valleys wider and deeper. Today, New York State’s deep, narrow lakes, the Finger Lakes, fill these glacially carved valleys and create some of the most beautiful landscapes in the United States.

In the region at the southern end of the glacier and beyond, deposition was more dominant than erosion during glaciation. Much of the inland lowlands are covered by glacial till, which consists of rocks and soil carried and dropped by the glacier, and can vary in thickness from less than a meter to more than 100 meters. In places where glaciers stopped flowing for long periods of time and stood still, they created high hill areas called moraines. In the East, Staten Island, Long Island, Martha’s Vineyard, and Cape Cod are on the edge of the moraine region, where the glaciers extended farthest to the southeast. The southern Great Lakes are punctuated by long, low, semicircular moraine ridges and other glacially deposited features.

There are some inland lowlands that have not been affected by glacial action. Southwestern Wisconsin and the 400-kilometer-long Mississippi River valley appear to have been spared the effects of glaciation because the deep valleys of Lakes Michigan and Superior, with the northern Superior Plateau acting as a barrier, redirected the glaciers. The result is a “driftless area,” an area of jagged boulders formed into natural piers or arches.

As the glaciers retreated, huge lakes were created along the edges of the glaciers. In the northern Great Plains, two huge lakes (Lake Agassi and Lake Lizaina) covered an area larger than the Great Lakes today. As the glaciers continued to retreat, most of these lakes disappeared. Evidence of these lakes can be seen in the flatlands that span parts of North Dakota and Minnesota, which were once lake beds.

During periods of widespread glaciation, sea levels were very low. This caused the rivers to become shallower and more eroded. The Susquehanna and Hudson rivers, in particular, cut through very deep valleys at the time. As the glaciers retreated, sea levels rose and seawater filled these deep valleys. This is how two of the world’s most beautiful harbor areas were formed. New York Bay and the Chesapeake Bay. The New York Bay connects to the deep Hudson River and is surrounded by a natural barrier formed by Staten Island and Long Island. The Chesapeake Bay is formed by the sinking of the Susquehanna River valley and its major tributaries, the Potomac and James Rivers.

In the eastern United States, the coastal plain narrows as one travels north along the coast to the Appalachian Plateau, where the lowlands disappear entirely at Cape Cod. From Cape Cod, the Appalachian Mountains rise northeastward to the ocean. The Appalachian Mountains, which span much of the eastern United States and were once high mountains, but have been eroded into their current state, separate the inland lowlands from the coastal areas.

The region’s generally very shallow soils and steep hills make modern farming methods, which emphasize the use of machinery, unsuitable. Large cities and large-scale industry developed primarily in the lowlands. Early settlers would have difficulty traveling west because the Appalachian Mountains stretch from the Mohawk River in New York State south to northern Alabama with few interruptions.

The western United States is a mountainous region with abrupt changes in elevation. The topography of the western United States consists of three main regions: the Rocky Mountains in the east, the mountains and valleys of the Pacific Coast, and the high, gaping plateau that separates them.

From the Great Plains, the Rocky Mountains to the west look like a giant face with peaks more than 2,000 kilometers above sea level. From other places, such as south-central Wyoming, the Rockies don’t seem to exist at all. The northern part of the Rockies, in Idaho, is a massive, dome-shaped igneous rock formation that runs north-south. This area contains some of the most vast wilderness left in the United States outside of Alaska.

The high plains of the western interior are also diverse in origin and shape. The southernmost Colorado Plateau is made of thick layers of sedimentary rock, rising more than 1,000 meters above the lowlands and gradually increasing in elevation to the northeast. The plateau is made up of spectacular canyons, volcanic peaks, and sandy deserts.

Further north is the Columbia-Snake Basin, which is filled with lava down to a depth of 1,000 meters. Throughout history, the Columbia and Snake rivers have eroded away rocks. The resulting landscape is similar to the Colorado Plateau, but without the time-carved, stepped appearance of the eroded sedimentary rocks of the Colorado Plateau. Volcanic craters can also be found in the region, especially in central southern Oregon and the Snake River Valley in Idaho.

The plateau gradually widens northward to include the Yukon River foothills of Alaska. In contrast, much of central Alaska consists of wide, flat lowlands with poor drainage.

The Pacific coast of the United States appears to be characterized by two mountain ranges that run north-south, with lowlands in between. The southern California side of the Coast Range is very large, with peaks rising to 3,000 meters. From there to the Oregon border, the Coast Range is low and straight, with few peaks rising above 1,000 meters. This area is also a major fault zone in the United States and is prone to earthquakes. The Klamath Mountains, along the California-Oregon state line, are taller and longer than the Coast Range, and much more sinuous and irregular. With the exception of the Olympic Mountains in northwestern Washington, the Coast Range, which spans the rest of Oregon and Washington, is too low and hilly to be considered mountainous.

The inland lowlands along the Pacific Coast-the Central Valley of California, the Willamette Valley of Oregon, and the Puget Sound lowlands of Washington-are the only large-scale lowlands near the West Coast. These lowlands have relatively high quality soils and support agriculture along the Pacific Coast.

To the east of the lowlands are the Sierra Nevada and Cascade mountain ranges. The Sierra Nevada Mountains look like a huge, faulted mass of land with a very sharp face pointing eastward and upward. While the western slopes of the Sierra Nevada Mountains are gentle, parts of the eastern range are over 3,000 meters high. Meanwhile, volcanic activity played a major role in shaping the Cascade Range. Two of the most famous volcanic peaks in the United States, Mount Rainier and Mount St. Helens in Washington, are located in the Cascade Mountains.

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