Afall line (orfall zone) is the area where an upland region and acoastal plain meet and is especially noticeable at the place rivers cross it, with resultingrapids orwaterfalls. The uplands are relatively hardcrystalline basement rock, and thecoastal plain is softersedimentary rock.[1] A fall line often will recede upstream as a river cuts out the uphill dense material, forming "c"-shaped waterfalls and exposing bedrockshoals. Due to these features, riverboats typically cannot travel any further inland withoutportaging, unlesslocks are built. The rapid change of elevation of the water and resulting energy release make the fall line a good location forwater mills,grist mills, andsawmills. Seeking ahead of navigation with a ready supply of water power, people have long made settlements where rivers cross a fall line.
The slope of rivers crossing fall zones affected settlement patterns. For example, the fall line represents the inland limit of navigation of many rivers. Numerous cities along a fall line grew as a result of demand for transferring people and goods between land-based and water-based transportation at that place.[2]
The Atlantic Seaboard Fall Line, is a 1,400-kilometre (900-mile)escarpment where thePiedmont andAtlantic Coastal Plain meet in the easternUnited States.[3] Much of the Atlantic Seaboard fall line passes through areas where no evidence offaulting is present.
The fall line marks the geologic boundary of hard metamorphosed terrain—- the product of theTaconic orogeny—- and the sandy, relatively flat outwash plain of the upper continental shelf, formed of unconsolidatedCretaceous andTertiarysediments. Examples of the Fall Zone include the Potomac River'sLittle Falls and the rapids inRichmond, Virginia, where theJames River falls across a series of rapids down to the tidal estuary of the James River.
TheLaurentian Upland forms a long scarp line where it meets theGreat Lakes–St. Lawrence Lowlands, just north of theSt. Lawrence river and estuary. Along this line, numerous rivers have carved falls and canyons (listed east to west):
TheRiver Jacques-Cartier andRiver Saint-Maurice lack such noticeable features because they cross the scarp throughU-shaped valleys. The falls of the lower Saint-Maurice (as well as those of the River Beauport, inQuebec City) are due to thefluvial terraces of theSaint Lawrence river rather than the Laurentian Scarp.
