The Coastal Plain
The Fall Line
The fall line is the boundary that divides the coastal plain from the piedmont to the west, so named for the waterfalls often present at this type of sharp elevation change. This scarp is the remnant of the Mesozoic Atlantic shoreline, when sea level was much higher and the coastline was further inland. As a result, the fall line is the interface between the softer sedimentary rocks of the coastal plain and the denser piedmont bedrock.
The fall line contains every major non-coastal city in the southern part of the east coast, with the exception of Charlotte; this is not by coincidence. The waterfalls on the steep slope of the fall line prevented any travel upriver from the coastal plain into the piedmont, so ports (which eventually became cities) were established there. The mill industry and trade of goods along the river gave these cities the wealth necessary to grow.
The flat and low topography of the coastal plain was formed by a series of these transgressions and regressions beginning in the Early Mesozoic which deposited layers of sediment on top of each other. Each layer in this wedge of sediments dips towards the coast, and the sedimentary deposits thicken overall as you move east. Since the sea level was changing, different types of sediment were deposited in what is now the modern coastal plain at different places at different times. The layers of deposited sediment with higher clay content act as confining units and divide the subterranean region into stratified aquifers (as seen below: the cross section of Virginia’s coastal plain).
These marine fluctuations are often evident in the sediment’s stratigraphy, due to the fact that different types of sediment are deposited in specific places when carried into the ocean by rivers. Deposition occurs by particle size, with the heaviest particles being deposited first in high-energy areas (closest to the shoreline). Sand is deposited first and ends up onshore (as beaches) and just offshore, followed by silt and then clay, the smallest sediment particles. Calcite (CaCO3) is deposited a few miles offshore from the remains of calcifying shellfish, corals, or coccolithophores. This is compacted over time, creating limestone.
Works consulted: Pilkey (1998)