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A fall line (or fall zone) is the area where an upland region and a coastal plain meet and is typically prominent where rivers cross it, with resulting rapids or waterfalls. The uplands are relatively hard crystalline basement rock, and the coastal plain is softer sedimentary rock. A fall line often will recede upstream as the river cuts out the uphill dense material, forming "c"-shaped waterfalls and exposing bedrock shoals. Because of these features, riverboats typically cannot travel any farther inland without portaging, unless locks are built. The rapid change in elevation of the water and resulting energy release make 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:
- Watertown, Massachusetts (Charles River), dam and mill built 1634
- Lowell, Massachusetts (Merrimack River)
- Hartford, Connecticut (Connecticut River)
- Fall River, Massachusetts (Quequechan River)
- Bangor, Maine (Penobscot River)
- Augusta, Maine (Kennebec River)
- Pawtucket, Rhode Island (Blackstone River)
- Atlantic Seaboard fall line (Mid-Atlantic and Southern fall lines):
- 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
- Washington, D.C., 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
- Weldon, North Carolina, and Roanoke Rapids, North Carolina, on the Roanoke River
- Rocky Mount, North Carolina, on the Tar River
- Raleigh, North Carolina, on the Neuse River
- Fayetteville, North Carolina, on the Cape Fear 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
Atlantic Seaboard Fall Line
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 Little 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 Little 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.
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