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The [[fall line]] marks the geologic boundary of hard metamorphosed terrain—the product of the [[Taconic orogeny]]—and the sandy, relatively flat [[alluvial plain]] of the upper continental shelf, formed of unconsolidated [[Cretaceous]] and [[Cenozoic]] [[sediment]]s. Examples of Fall Zone features include the [[Little Falls (Potomac River)|Potomac River's Little Falls]] and the rapids in [[Richmond, Virginia]], where the [[James River]] falls across a series of rapids down to its own tidal estuary.
Before navigation improvements, such as locks, the fall line was generally the [[wikt:head of navigation|head of navigation]] on rivers due to their rapids or waterfalls, and the necessary [[portage]] around them. Numerous cities initially formed along the fall line because of the easy river transportation to seaports, as well the availability of water power to operate mills and factories, thus bringing together river traffic and industrial labor. [[U.S. Route 1]] and [[Interstate 95|I-95]] link many of the fall-line cities.
In 1808, [[United States Secretary of the Treasury|Treasury Secretary]] [[Albert Gallatin]] noted the significance of the fall line as an obstacle to improved national communication and commerce between the Atlantic seaboard and the western river systems:<ref>[Report on] Roads and Canals, Communicated to the Senate April 4, 1808, [http://memory.loc.gov/cgi-bin/ampage?collId=llsp&fileName=037/llsp037.db&recNum=736 p.729]</ref>
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