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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