The dominant topographic features of the United States tend to extend north-south across the country. The interior of the country is a vast, sprawling lowland that stretches from the Gulf of Mexico to the Canadian border and then on to Alaska. Geographers with an interest in landform development place this expanse of flat land and gently rolling hills in three different physiographic regions--the Atlantic and Gulf coastal plains, the interior lowland (which some split into the Great Plains and the interior plains), and the Canadian Shield.
The Atlantic and Gulf coastal plains reach north along the east coast of the United States as far as the southern margins of New England. Underlying this area are beds of young, soft, easily eroded rock deposited in recent geologic time as shallow seas lapped back and forth across the land. These low plains extend well out under the ocean surface to form a continental shelf, which in places extends as much as 400 kilometers beyond the shore.
Northward, the interior lowland, although noticeably hillier than the coastal plains, has almost no rough terrain. This region is like a saucer, turned up at the edges and covered with a deep series of sedimentary rocks. These sedimentary beds are generally quite flat; most topographic variation is the result of local erosion or, in the North, of glacial debris deposited during the Ice Age.
The geologic structure of the Great Plains differs little from that of the interior plains. The sedimentary beds dominate, although in the north they are broken by some eroded domes, most notably the Black Hills of western South Dakota. While nearly horizontal, the sedimentary beds do dip gently toward the west to a trough at the foot of the Rocky Mountains, where the Colorado cities of Denver and Colorado Springs are located.
The boundary between the Great Plains and the interior plains is marked by a series of low escarpments that indicate the eastern edge of the mantle of loose sediments, eroded from the Rocky Mountains, that covers the plains.
The character of this massive interior lowland area has had a number of important influences on the economic and settlement history of the United States. In addition to the vast agricultural potential it provides, fully half the country can be crossed without encountering significant topographic barriers to movement. This facilitated the integration of both this region and the distant West into the economic fabric of the country. Nearly all of the interior lowland is drained by the Mississippi River or its tributaries. This drainage pattern assisted regional integration by providing a transport and economic focus for the land west of the Appalachian Mountains.
North and northeast of the central lowland is the Canadian Shield, where old, hard crystalline rocks lie at the surface. Farther south in the lowlands, similar rocks are covered by the sedimentary beds deposited under the sea that once filled the midsection of the country. Erosion has worn down the surface of the Shield into a lowland of small local relief.
The Shield, more than any other North American physiographic region, has had its landforms remolded and shaped by massive continental glaciers during the last million years. These glaciers covered most of Canada east of the Rocky Mountains and the Coast Ranges, and they reached southward to approximately the present valleys of the Missouri and Ohio Rivers.
The ice could pluck rocks weighing many tons off the surface and carry them great distances: Massive boulders are strewn across the landscape of the Shield, resting where they were dropped by the glaciers. Ice melt along the peripheries of the glaciers created major rivers and cut broad new pathways to the sea.
Glaciation scoured much of the Shield's surface. Today, the soil cover of the region remains thin or nonexistent. The heavily disrupted drainage pattern dammed many streams with debris and led others into the area's labyrinth of lakes and swamps rather than to the sea. Central and northern Minnesota, for example, called the "Land of 10,000 Lakes," is part of the southern lobe of the glaciated shield that extends into the states of Minnesota, Michigan, and Wisconsin.
Southward, where the ice was not as thick and its force correspondingly less, the glaciers were diverted or channeled by higher elevations. For example, the ice was blocked in central New York by the highlands south of the Mohawk River. However, narrow probes did push up the valleys of streams tributary to the Mohawk, gradually broadening and deepening them. Today, the deep, narrow Finger Lakes of New York State fill these glacially enlarged valleys and form one of America's truly beautiful landscapes.
All along and beyond the southern edges of the glaciers, deposition replaced erosion as the prime result of glaciation. Large areas of the interior lowland are covered by a mantle of glacial till (rocks and soil dropped by the glaciers), which covers the land to depths varying from a meter or less to more than 100 meters. Where the glaciers remained stationary for long periods of time, higher hills, called moraines, were created. In the east, Staten Island, Long Island, Martha's Vineyard, Nantucket, and Cape Cod are end moraines that mark the farthest major extension of glaciers toward the southeast. The landscape south of the Great Lakes is laced with long, low, semicircular moraine ridges and other glacial deposits.
One section of the interior lowland escaped glaciation. The southwestern quarter of Wisconsin and the adjoining 400-kilometer stretch of the Mississippi River valley were apparently spared by the barrier effect on the flowing ice of the Superior upland to the north and by the channeling of the ice by the deep valleys of Lakes Michigan and Superior. The result is the "driftless area" (drift is another name for till), a local landscape that is more angular, with fragile rock formations like natural bridges and arches.
As the ice retreated, massive lakes were created along the glacial margins. On the northern Great Plains, two huge lakes, Agassiz and Regina, together covered an area larger than today's Great Lakes. With continued glacial retreat, these lakes mostly disappeared. Their existence is now marked by the former lake bed, a flat area covering parts of North Dakota and Minnesota.
Sea level was significantly lower during periods of widespread glaciation. This lowered the base level of many rivers and thus fostered increased erosion by those streams. Furthermore, many of these stream valleys extended well into what is today the ocean. Along with many others, the Susquehanna and Hudson Rivers cut much deeper valleys during this period. As the ice retreated and sea level rose, the ocean filled these deepened valleys. Two of the world's finest harbor areas were formed in this way: New York Bay, with the deep Hudson River and the protective barriers formed by Staten Island and Long Island; and Chesapeake Bay, the drowned valley of the Susquehanna River and some of its major former tributaries, such as the Potomac and James Rivers.
In the East, the coastal plains are gradually squeezed against the coast northward along the ocean by the Appalachian Highlands until the lowland disappears entirely at Cape Cod. From there northeastward, the coastal landscape is a part of the northern extension of the Appalachian Mountain system. The Appalachians--eroded remnants of what were once much higher mountain ranges--separate the seaboard from the interior lowlands along much of the eastern United States.
Soils in most parts of this region are shallow, and the steep slopes, difficult to farm under any circumstances, are totally unsuited to modern agricultural practices that emphasize mechanization. Large-scale urban or industrial growth is cramped by the small, local lowlands. Early settlers found the Appalachians from the Mohawk River in New York southward to northern Alabama to be a surprisingly effective barrier to western movement; there are few breaks in the mountains' continuity.
The western United States is a land of mountains and of sudden, great changes in elevation. The physiography, again, is arranged in a series of three large north-south trending bands, with the Rocky Mountains on the east separated from the mountains and valleys of the Pacific coastlands by a series of high, heavily dissected plateaus.
Starting in the east, the Rocky Mountains generally present a massive face to the Great Plains, with peaks occasionally rising 2 kilometers or more. Elsewhere, as in south-central Wyoming, the Rockies almost seem not to exist at all. In the northern Rockies in Idaho, the north-south linearity of most of the region's mountains is replaced by massive igneous domes irregularly eroded into a rugged, extensive series of mountain ranges that contain the largest remaining area of wilderness in the United States outside Alaska.
The high plateaus of the interior West are also varied in their origin and appearance. The southernmost subsection, the Colorado Plateau, is a series of thick beds of sedimentary rocks rising more than 1,000 meters above the lowlands' elevation and tilted upward toward the northeast. The plateau is a land of spectacular canyonlands, volcanic peaks, and sandy deserts.
Farther north, the Columbia-Snake Basin has been filled by repeated lava flows to a depth of more than 1,000 meters. Rivers, both past and present, have eroded into the rock. The resultant landscape is similar to that of the Colorado Plateau, although the stepped appearance resulting from the variable resistance to weathering of the eroded sedimentary rocks of the Colorado Plateau is missing. Volcanic cones also dot portions of the region, especially across south-central Oregon and in the Snake River Valley in Idaho.
The plateaus gradually widen northward, encompassing the valley of the Yukon River in Alaska. In comparison, much of central Alaska is a broad, flat lowland that is poorly drained.
In the conterminous United States (excluding Alaska and Hawaii), the Pacific Coast seems to consist largely of two north-south trending mountain chains separated by a discontinuous lowland. In southern California, the Coast Range is fairly massive, with peaks reaching 3,000 meters. From there almost to the Oregon border, the mountains are low and linear, seldom rising above 1,000 meters. This also is the major fault zone of the state and a region of frequent earthquake activity. Along the California-Oregon border, the Klamath Mountains are higher, more extensive, and much more rugged and irregular. Except for the Olympic Mountains in northwestern Washington, the Coast Ranges in the rest of Oregon and Washington State are low and hilly rather than mountainous.
The interior lowlands along the coast--the Central Valley of California, the Willamette Valley in Oregon, and the Puget Sound lowland in Washington--are the only extensive lowlands near the West Coast. Filled with relatively good soils, these lowlands have supported much of the Pacific Coast's agriculture.
East of the lowlands are the Sierra Nevada and the Cascade mountain ranges. The Sierra Nevada appears as though a massive section of earth was tilted upward relative to the areas to the east and west in what is called a fault block, with the highest, sharpest exposed face toward the east. Although the western approaches into the Sierra Nevada are reasonably gentle, on the eastern side the mountains rise in some places more than 3,000 meters. Volcanic activity was important in the formulation of the Cascades. Some of America's best known volcanic peaks, such as Mt. Rainier and Mt. St. Helens in Washington, are found there.
Source: U.S. Department of State