This regional view shows the striking visual effect of the valley-and-ridge topography of the Appalachian Mountains as viewed from the International Space Station. The image shows more than 500 kilometers (300 miles) of this low mountain chain from northeast Pennsylvania (top right) to southern West Virginia, where a dusting of snow covers a patch of land (lower left).
Sunglint reflections reveal details of Chesapeake Bay and the great bend of the Potomac River. Cities are difficult to detect from space during daylight hours, so the sickle-shaped bend of the river is a good visual guide for astronauts trying to photograph the nation’s capital, Washington D.C. The farm-dominated Piedmont Plateau is the light-toned area between the mountains and the bay.
The Appalachian Mountains appear striped because the ridges are forested, providing a dense and dark canopy cover, while the valleys are farmed with crops that generally appear as lighter-toned areas. (Farmland is even lighter than usual in this image because the fields are fallow after the harvest.) Geologically, the valleys are the softer, more erodible rock layers, much the preferred places for human settlement. Not only do the larger rivers occupy the valley floors, but the soils are thicker, the slopes are gentler, and the valleys are better protected from winter winds.
The rocks that form this valley-and-ridge province, as it is known, are relatively old (540 to 300 million years old) and were laid down in horizontal layers when North America was attached to Europe in the ancient supercontinent of Laurasia. During this time Gondwanaland—an ancient supercontinent that included present-day Africa, India, South America, Australia, and Antarctica—was approaching Laurasia under the influence of plate tectonics. The northwest coastline of modern Africa was the section of Gondwanaland that “bumped up” against modern North America over a long period (320 to 260 million years ago).
The net result of the tectonic collision was the building of a major mountain chain, much higher than the present Appalachian range. In the process, the flat-lying rock layers were crumpled up into a series of tight folds, at right angles to the advance of Gondwanaland. The collision also formed the singular supercontinent of Pangaea. Over the following 200 million years, Pangaea broke apart; the modern Atlantic Ocean formed; and erosion wore down the once-high mountains. What is left 200 million years later are the coastline of North America and the eroded stumps of the relatively low, but visually striking mountain chain.