fall line, boundary between an upland region and a coastal plain across which rivers from the upland region drop to the plain as falls or rapids. A fall line is formed in an area where the rivers have eroded away the soft rocks of a coastal plain more quickly than the older harder rocks of an upland region. Such erosion follows a crooked line along a coast. River vessels usually cannot travel beyond a fall line and their cargoes must be unloaded there. The falls (see waterfall) also supply water power for the development of industry such as textile and grist mills. For these reasons a fall line often marks a string of developed areas, such as the break between the Appalachian rise and the coastal plain of the eastern United States, where a band of commercial and industrial cities quickly developed in the 19th cent., paralleling the line of port cities along the coasts. Typical fall-line cities on the Atlantic coast of the United States are Lowell, Mass.; Pawtucket, R.I.; Troy, N.Y.; Trenton, N.J.; Georgetown, now part of Washington, D.C.; Richmond, Va.; Raleigh, N.C.; Columbia, S.C.; and Augusta, Ga. Among the fall-line cities of the Mississippi valley are Louisville, Ky., and Minneapolis, Minn.
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