||It has been suggested that Fall Line Cities be merged into this article. (Discuss) Proposed since May 2012.|
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, fall lines proved useful for hydroelectric dams such as at Rochester, New York (on the Niagara Escarpment) and Columbia, South Carolina (on the Atlantic Seaboard fall line). Other cities along fall lines of the United States include:
|Atlantic Seaboard fall line|
|Nickname: Piedmont—Coastal Plain fall line|
Carolinas or Georgia 
|Length||900 mi (1,400 km) |
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.
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