United States

The United States is relatively young by world standards, being less than 250 years old; it achieved its current size only in the mid-20th century. America was the first of the European colonies to separate successfully from its motherland, and it was the first nation to be established on the premise that sovereignty rests with its citizens and not with the government. In its first century and a half, the country was mainly preoccupied with its own territorial expansion and economic growth and with social debates that ultimately led to civil war and a healing period that is still not complete. In the 20th century the United States emerged as a world power, and since World War II it has been one of the preeminent powers. It has not accepted this mantle easily nor always carried it willingly; the principles and ideals of its founders have been tested by the pressures and exigencies of its dominant status. The United States still offers its residents opportunities for unparalleled personal advancement and wealth. However, the depletion of its resources, the contamination of its environment, and the continuing social and economic inequality that perpetuates areas of poverty and blight all threaten the fabric of the country.

The District of Columbia is discussed in the article Washington. For discussion of other major U.S. cities, see the articles Boston, Chicago, Los Angeles, New Orleans, New York City, Philadelphia, and San Francisco. Political units in association with the United States include Puerto Rico, discussed in the article Puerto Rico, and several Pacific islands, discussed in Guam, Northern Mariana Islands, and American Samoa. The Editors of Encyclopaedia Britannica

Land

The two great sets of elements that mold the physical environment of the United States are, first, the geologic, which determines the main patterns of landforms, drainage, and mineral resources and influences soils to a lesser degree, and, second, the atmospheric, which dictates not only climate and weather but also in large part the distribution of soils, plants, and animals. Although these elements are not entirely independent of one another, each produces on a map patterns that are so profoundly different that essentially they remain two separate geographies. (Since this article covers only the conterminous United States, see also the articles Alaska and Hawaii.)

The Colorado River in Marble Canyon at the northeastern end of Grand Canyon National Park, northwestern Arizona.© Gary Ladd

Relief

The centre of the conterminous United States is a great sprawling interior lowland, reaching from the ancient shield of central Canada on the north to the Gulf of Mexico on the south. To east and west this lowland rises, first gradually and then abruptly, to mountain ranges that divide it from the sea on both sides. The two mountain systems differ drastically. The Appalachian Mountains on the east are low, almost unbroken, and in the main set well back from the Atlantic. From New York to the Mexican border stretches the low Coastal Plain, which faces the ocean along a swampy, convoluted coast. The gently sloping surface of the plain extends out beneath the sea, where it forms the continental shelf, which, although submerged beneath shallow ocean water, is geologically identical to the Coastal Plain. Southward the plain grows wider, swinging westward in Georgia and Alabama to truncate the Appalachians along their southern extremity and separate the interior lowland from the Gulf.

West of the Central Lowland is the mighty Cordillera, part of a global mountain system that rings the Pacific basin. The Cordillera encompasses fully one-third of the United States, with an internal variety commensurate with its size. At its eastern margin lie the Rocky Mountains, a high, diverse, and discontinuous chain that stretches all the way from New Mexico to the Canadian border. The Cordillera’s western edge is a Pacific coastal chain of rugged mountains and inland valleys, the whole rising spectacularly from the sea without benefit of a coastal plain. Pent between the Rockies and the Pacific chain is a vast intermontane complex of basins, plateaus, and isolated ranges so large and remarkable that they merit recognition as a region separate from the Cordillera itself.

These regions—the Interior Lowlands and their upland fringes, the Appalachian Mountain system, the Atlantic Plain, the Western Cordillera, and the Western Intermontane Region—are so various that they require further division into 24 major subregions, or provinces.

The Interior Lowlands and their upland fringes