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In geomorphology, a fall line marks the area where an upland region (continental bedrock) and a coastal plain (coastal alluvia) meet. Technically, a fall line is an unconformity. A fall line is typically prominent when crossed by a river, for there will often be rapids or waterfalls. Because of these features river boats typically cannot travel any farther inland without portaging unless locks are built. Because of the need for a port and a ready supply of water power, settlements often develop where rivers cross a fall line.

Along the eastern coast of the United States, the east-facing escarpment where the Piedmont of the Appalachians descends steeply to the coastal plain forms a fall line over 1500 kilometers long. This long fall line played a major role in settlement patterns along rivers, back into prehistoric times. It is often referred to simply as "the fall line". In some places the fall line may be abrupt, while in others it is a zone that may be many miles wide. Geologically the fall line marks the 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.

In the 19th Century, the fall line often represented the head of navigation on rivers at points like Little Falls or the Great Falls, on the Potomac River. However, since the advent of flumes for water supply and canals for shipping in the early 20th Century, the most prominent feature of fall line settlement was the establishment of the cities along it. As the cities were linked by the early highways, U.S. Route 1 and Interstate 95 came to pass through many of these cities, roughly tracing the fall line.

Cities along the Piedmont – Coastal Plain fall line include, from north to south:

• Paterson, New Jersey on the Passaic River (Great Falls of the Passaic River)
• New Brunswick, New Jersey on the Raritan River
• Trenton, New Jersey on the Delaware River
• Philadelphia, Pennsylvania on the Schuylkill River
• Wilmington, Delaware on the Brandywine Creek
• Conowingo, Maryland on the Susquehanna River
• Baltimore, Maryland on the Patapsco River
• Laurel, Maryland on the Patuxent River
• Washington, D.C./Georgetown/Alexandria on the Potomac River (Great Falls National Park)
• Occoquan, Virginia on the Occoquan River
• Fredericksburg, Virginia on the Rappahannock River
• Richmond, Virginia on the James River
• Petersburg, Virginia on the Appomattox River
• Roanoke Rapids, North Carolina on the Roanoke River
• Smithfield, North Carolina on the Neuse 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
• Auburn, Alabama
• Tuscaloosa, Alabama on the Black Warrior River

Cities along other fall lines include:

• Lowell, Massachusetts on the Merrimack River
• Hartford, Connecticut on the Connecticut River
• Albany, New York on the Hudson River

• Escarpment
• Fall Line Freeway

• USGS: The Fall Line: A Tapestry of Time and Terrain
• Virginia Places website: The Fall Line
• Virginia Places website: Rivers and Fall Line Cities provides a more detailed explanation of why some towns and cities along a Fall line grew and others did not.
• The Geology of Georgia