The Atlantic Seaboard Fall Line, or Fall Zone, is a 900-mile (1,400Â km) 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 terrainâ€"the product of the Taconic orogenyâ€"and the sandy, relatively flat outwash plain of the upper continental shelf, formed of unconsolidated Cretaceous and Cenozoic sediments. Examples of the Fall Zone 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. The Little Falls of the Potomac River is one example. Because of the commercial traffic, required labor and availability of water power to operate mills, numerous cities were founded at the intersection of rivers and the fall line. U.S. Route 1 links 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, through which the rivers have forced their passage...
Notable cities
Some cities that lie along the Piedmontâ€"Coastal Plain fall line include the following (from north to south):
- Trenton, New Jersey on the Delaware River.
- Philadelphia, Pennsylvania on the Schuylkill River.
- Smyrna, Delaware near Bombay Hook National Wildlife Refuge.
- Perryville, Maryland and Havre de Grace, Maryland on the Susquehanna River/head of Chesapeake Bay.
- Baltimore, Maryland, on Herring Run, Jones Falls, and Gwynns Falls.
- Elkridge, Maryland on the Patapsco River.
- Laurel, Maryland on the Patuxent River.
- Washington, D.C. on the Potomac River.
- Occoquan, Virginia on the Occoquan River.
- Fredericksburg, Virginia on the Rappahannock River.
- Richmond, Virginia on the James River.
- Petersburg, Virginia on the Appomattox River.
- Weldon, North Carolina, and Roanoke Rapids, North Carolina, on the Roanoke River
- Raleigh, North Carolina on the Neuse River via Falls Lake.
- Greenville, North Carolina on the Tar River.
- Fayetteville, North Carolina on the Cape Fear River.
- Cheraw, South Carolina on the Pee Dee River.
- Camden, South Carolina on the Wateree River.
- Columbia, South Carolina on the Congaree River.
- Augusta, Georgia on the Savannah River.
- Milledgeville, Georgia on the Oconee River.
- Macon, Georgia on the Ocmulgee River.
- Columbus, Georgia on the Chattahoochee River.
- Tallassee, Alabama, on the Tallapoosa River
- Wetumpka, Alabama, on the Coosa River
- Tuscaloosa, Alabama, on the Black Warrior River
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