A fall line (or fall zone) is the geomorphologic break that demarcates the border 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 and exposing bedrock shoals. 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.
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.
^"The Fall Line". A Tapestry of Time and Terrain: The Union of Two Maps - Geology and Topography. USGS.gov. Archived from the original on 2011-05-14. Retrieved 2010-08-12. An alternate source claims the southern endpoint is farther west because there are "waterfalls & rapids":
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