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A fall line (or fall zone) is the geomorphologic break  between an upland region of relatively hard crystalline basement rock and a coastal plain of softer sedimentary rock. A fall line is typically prominent when crossed by a river, for there will often be rapids or waterfalls. Many times a fall line will recede upstream as the river cuts out the uphill dense material, often forming “c”-shaped waterfalls. Because of these features riverboats typically cannot travel any farther inland without portaging, unless locks are built there. On the other hand, the rapid change in elevation of the water, and the resulting energy release, makes the fall line a good location for water mills, grist mills, and sawmills. Because of the need for a river port leading to the ocean, and a ready supply of water power, settlements often develop where rivers cross a fall line.
The slope of fall zones on rivers played a role in settlement patterns. For example, the fall line represents the inland limit of navigation on many rivers. As such, many fall line cities grew around transferring people and goods between land-based and water-based transportation at this point. Also, fall lines proved useful for hydroelectric dams such as those at Rochester, New York (on the Niagara Escarpment), at Columbia, South Carolina, and at Conowingo, Maryland, on the Susquehanna River (on the Atlantic Seaboard fall line). Cities established along fall lines in the United States include:
- New England:
- Atlantic Seaboard fall line (Southern fall line):
- Trenton, New Jersey, on the Delaware River
- Paterson, New Jersey, on the Passaic River
- Philadelphia, Pennsylvania, on the Schuylkill River
- Wilmington, Delaware, on Brandywine Creek
- Baltimore, Maryland, on the Jones Falls, Gunpowder Falls and Gwynns Falls
- Great Falls, Virginia, on the Potomac River
- Fredericksburg, Virginia, on the Rappahannock River
- Hanover, Virginia, on the North Anna River
- Richmond, Virginia, on the James River
- Petersburg, Virginia, on the Appomattox River
- Roanoke Rapids, North Carolina, on the Roanoke River
- Raleigh, North Carolina, on the Neuse River
- Fayetteville, North Carolina, on the Cape Fear 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
Atlantic Seaboard Fall Line
|Atlantic Seaboard fall line|
|Nickname: Piedmont—Coastal Plain fall line|
Carolinas or Georgia 
|Length||900 mi (1,400 km) |
Map showing part of the Eastern Seaboard Fall Line where the pale colored coastal plain meets the brightly colored Piedmont.
The Atlantic Seaboard Fall Line, or Fall Zone, is a 900-mile (1,400 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 terrain—the product of the Taconic orogeny—and the sandy, relatively flat outwash plain of the upper continental shelf, formed of unconsolidated Cretaceous and Tertiary sediments. Examples of the Fall Zone include the Potomac River's Great Falls and the rapids in Richmond, Virginia, where the James River falls across a series of rapids down to the tidal estuary of the James River. Columbia, South Carolina is similar as well with the Congaree River.
Before navigation improvements such as locks, the fall line was often the head of navigation on rivers due to rapids and waterfalls, such as the Great Falls of the Potomac River. Numerous cities were founded at the intersection of rivers and the fall line. U.S. Route 1 links many of the fall line cities.
- McGee, W.J. (American physiographer) (1888), The Geology of the Head of Chesapeake Bay, Geological Survey 7th Annual Report, pp. 537–646
- Schneider, Craig W.; Richard B. Searles (1991). Seaweeds of the southeastern United States: Cape Hatteras to Cape Canaveral. Duke University Press. pp. 5–7. ISBN 978-0-8223-1101-0. Retrieved 17 November 2010.
- "The Fall Line and major cities of the Eastern U.S.".
- "Fall Line". The New Georgia Encyclopedia. Retrieved 17 October 2011.
- Freitag, Bob; Susan Bolton; Frank Westerlund; Julie Clark (2009). Floodplain Management: A New Approach for a New Era. Island Press. p. 77. ISBN 978-1-59726-635-2. Retrieved 17 November 2010.
- "The Fall Line". A Tapestry of Time and Terrain: The Union of Two Maps - Geology and Topography. USGS.gov. Retrieved 2010-08-12. An alternate source claims the southern endpoint is farther west because there are "waterfalls & rapids":
- "Georgia Geology". Retrieved 2010-08-13.