The present-day pattern of the landforms of the United States is the result of a long sequence of collisions and separations of large blocks of the Earth’s surface crust, a process known as plate tectonics. The oldest part of the continent is the Canadian Shield, or Laurentian Plateau, a mass of granite and related rock that underlies eastern Canada and the north-eastern United States. The shield was formed during several long periods of crustal convergence in Precambrian time (a period that stretches from the formation of the Earth to about 570 million years ago). The characteristic rock of the shield is granite. The margins of the ancient continent are more complex in structure, and include zones of granite, darker ocean-bottom rocks, fine-grained volcanic rocks, and hardened ocean sediments.
A long period of inactivity in the crust followed the formation of the shield. Erosion reduced the mountainous continent to a low plain, and the adjoining seas were filled with thick beds of sediment. Near the end of this period, great forests covered the land, and the addition of organic material to the sediment formed the vast coal and petroleum layers that stretch in a broad curve from northern Pennsylvania through West Virginia to Alabama, then west to Texas and north-west through the Great Plains states and Canadian prairies to Arctic Alaska.
The period of crustal calm ended when the North American and European land masses collided early in the fossil-forming period; southern Massachusetts and Rhode Island are actually parts of the European land mass that became attached to the American plate at this time.
Later, after the coal-forming age, the African and American land masses converged. The modern Appalachians are the worn-down remnants of the mountains that were built during this collision. Crustal uplift and subsequent erosion exposed ancient granite rocks all the way from New England to Alabama, as evidenced in the low Piedmont hills of Georgia and the Carolinas and the somewhat higher Blue Ridge Mountains of North Carolina, Virginia, and West Virginia. To the west, the layers of younger sedimentary rocks still remain at the surface, crumpled and eroded, notably in the long, even ridges and valleys of central Pennsylvania, eastern Tennessee, and the Ouachita region of Arkansas and Oklahoma. Still further west and north, less intense folding created broad domes and basins. Present-day Michigan and Iowa occupy geological basins. Structural domes are centred near the Wisconsin Dells, the Bluegrass area of Kentucky, and the Nashville area of Tennessee.
After the Appalachian collision, the continent reversed direction and drifted west. The Atlantic Ocean began to widen, and the eastern United States again became a region of geological calm. The Appalachians began to erode, and the resulting sediment accumulated on the mid-continental Great Plains and on the Atlantic and Gulf coastal plains. Meanwhile, new ranges of mountains were rising as the western United States collided with the Pacific plate. Lava erupted on to the surface in many places at different times: in northern New Mexico, central Arizona, eastern California, and southern Idaho, and especially in the region of the Cascade Range of Oregon and Washington. The sandy sediments of the Great Plains were thrust sharply upward along the Front Range of the Rocky Mountains in Colorado and Montana and around smaller mountain ranges such as the Black Hills of South Dakota. Rock movement along massive faults formed California’s Sierra Nevada, the Wasatch Range in Utah, the aligned mountain ranges of Nevada, and the Teton Range in Wyoming. The land of Arizona and southern Utah was lifted, and rivers cut canyons into the level sedimentary rocks.
The major past climatic event is the Pleistocene epoch, more commonly known as the Ice Age. At least four times in the past 1 million years, great ice sheets formed in eastern Canada and the mountains of the west and spread outward. The moving ice scraped up soil and rock from Canada and the northern United States and deposited the material further south.
The aligned lakes and exposed rocks of New England and northern Minnesota are the result of glacial scouring. Long Island and Cape Cod are huge glacial deposits, characterized by hills composed of rock and soil, with associated swamps and sand outwash plains; similar features are abundant throughout the former glaciated areas, from New England to the Dakotas and in the western mountain valleys. Rivers such as the Hudson, Illinois, Minnesota, Missouri, and Columbia carried huge floods of glacial meltwater and carved valleys much larger than the present-day streams require. Glacial meltwater also formed many large lakes. Today, level plains and low beach ridges mark the beds and shores respectively of Ice Age lakes on both the eastern and western edges of Vermont, around the Maumee River of north-western Ohio, in the sand counties of central Wisconsin, around the Red River of Minnesota and the Dakotas, around the Great Salt Lake of Utah, and in the Missoula Basin of Montana, and the Central Valley of California. Ice Age dust storms left thick deposits of loess (fine-grained silt or clay) on the undulating plains around the Mississippi and Missouri rivers, on the steeper bluffs of western Wisconsin and western Tennessee, and in the Palouse Hills region of eastern Washington. Times of higher sea level built beaches far up on the Gulf Coastal Plain and on slopes overlooking the Pacific Ocean, while Chesapeake Bay and many similar drowned river valleys along the Atlantic coast from Georgia to Connecticut are the result of periods of lower sea level. "USA" © Emmanuel BUCHOT, Encarta, Wikipedia
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