Atlantic Seaboard Fall Line

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The Atlantic Seaboard Fall Line, or Fall Zone, is a 900 mi escarpment where the Piedmont and Atlantic coastal plain meet in the eastern United States. Much of the Atlantic Seaboard fall line passes through areas where no evidence of faulting is present.

The fall line marks the geologic boundary of hard metamorphosed terrain—the product of the Taconic orogeny—and the sandy, relatively flat alluvial plain of the upper continental shelf, formed of unconsolidated Cretaceous and Cenozoic sediments. Examples of Fall Zone features include the Potomac River's Little Falls and the rapids in Richmond, Virginia, where the James River falls across a series of rapids down to its own tidal estuary.

Before navigation improvements, such as locks, the fall line was generally the head of navigation on rivers due to their rapids or waterfalls, and the necessary portage around them. Numerous cities initially formed along the fall line because of the easy river transportation to seaports, as well as the availability of water power to operate mills and factories, thus bringing together river traffic and industrial labor. U.S. Route 1 and Interstate 95 link many of the fall-line cities.

In 1808, Treasury Secretary Albert Gallatin noted the significance of the fall line as an obstacle to improved national communication and commerce between the Atlantic seaboard and the western river systems:

The most prominent, though not perhaps the most insuperable obstacle in the navigation of the Atlantic rivers, consists in their lower falls, which are ascribed to a presumed continuous granite ridge, rising about one hundred and thirty feet above tide water. That ridge from New York to James River inclusively arrests the ascent of the tide; the falls of every river within that space being precisely at the head of the tide; pursuing thence southwardly a direction nearly parallel to the mountains, it recedes from the sea, leaving in each southern river an extent of good navigation between the tide and the falls. Other falls of less magnitude are found at the gaps of the [Blue Ridge Mountains

Blue Ridge]], through which the rivers have forced their passage...

Gallatin's observation was sound, though simplified and limited by the knowledge of his time. The limits of the Fall Line are subject to some dispute. In the north, the fall line is usually understood to have its northern limit at [New Brunswick, New Jersey, a geologic continuation in fact crosses the Hackensack and Passaic rivers at the cities of those names, to which navigation was possible. In the south, some such as Gallatin above, and the USGS source in the infobox, imply its end to be in the Carolinas or Georgia, and to include only rivers running to the Atlantic; but it is more accurate, as the Georgia source in the infobox does, to trace it farther west through Georgia and Alabama, as that is the geologic continuation.