Читаем United States полностью

West of the crystalline zone, away from the axis of primary geologic deformation, sedimentary rocks have escaped metamorphism but are compressed into tight folds. Erosion has carved the upturned edges of these folded rocks into the remarkable Ridge and Valley country of the western Appalachians. Long linear ridges characteristically stand about 1,000 feet (300 metres) from base to crest and run for tens of miles, paralleled by broad open valleys of comparable length. In Pennsylvania, ridges run unbroken for great distances, occasionally turning abruptly in a zigzag pattern; by contrast, the southern ridges are broken by faults and form short, parallel segments that are lined up like magnetized iron filings. By far the largest valley—and one of the most important routes in North America—is the Great Valley, an extraordinary trench of shale and limestone that runs nearly the entire length of the Appalachians. It provides a lowland passage from the middle Hudson valley to Harrisburg, Pennsylvania, and on southward, where it forms the Shenandoah and Cumberland valleys, and has been one of the main paths through the Appalachians since pioneer times. In New England it is floored with slates and marbles and forms the Valley of Vermont, one of the few fertile areas in an otherwise mountainous region.

ridge-and-valley topographyRidge-and-valley topography in the Appalachian Highlands, near Bristol, Tennessee, U.S.Bantosh

Topography much like that of the Ridge and Valley is found in the Ouachita Mountains of western Arkansas and eastern Oklahoma, an area generally thought to be a detached continuation of Appalachian geologic structure, the intervening section buried beneath the sediments of the lower Mississippi valley.

The once-glaciated New England section of the Appalachians is divided from the rest of the chain by an indentation of the Atlantic. Although almost completely underlain by crystalline rocks, New England is laid out in north–south bands, reminiscent of the southern Appalachians. The rolling, rocky hills of southeastern New England are not dissimilar to the Piedmont, while, farther northwest, the rugged and lofty White Mountains are a New England analogue to the Blue Ridge. (Mount Washington, New Hampshire, at 6,288 feet [1,917 metres], is the highest peak in the northeastern United States.) The westernmost ranges—the Taconics, Berkshires, and Green Mountains—show a strong north–south lineation like the Ridge and Valley. Unlike the rest of the Appalachians, however, glaciation has scoured the crystalline rocks much like those of the Canadian Shield, so that New England is best known for its picturesque landscape, not for its fertile soil.

Mount Washington, in the White Mountains, New Hampshire.William Hemmel/© New Hampshire Division of Travel and Tourism Development

Typical of diverse geologic regions, the Appalachians contain a great variety of minerals. Only a few occur in quantities large enough for sustained exploitation, notably iron in Pennsylvania’s Blue Ridge and Piedmont and the famous granites, marbles, and slates of northern New England. In Pennsylvania the Ridge and Valley region contains one of the world’s largest deposits of anthracite coal, once the basis of a thriving mining economy; many of the mines are now shut, oil and gas having replaced coal as the major fuel used to heat homes.

The Atlantic Plain

The eastern and southeastern fringes of the United States are part of the outermost margins of the continental platform, repeatedly invaded by the sea and veneered with layer after layer of young, poorly consolidated sediments. Part of this platform now lies slightly above sea level and forms a nearly flat and often swampy coastal plain, which stretches from Cape Cod, Massachusetts, to beyond the Mexican border. Most of the platform, however, is still submerged, so that a band of shallow water, the continental shelf, parallels the Atlantic and Gulf coasts, in some places reaching 250 miles (400 km) out to sea.