If you’re looking for a new place to live on the East Coast of the US, and you dig on the dogs, it’s not a bad idea to pay attention to the Fall Line, the demarcation line between the Piedmont (literally rock-mountain) and the coastal plain.
I live just below Great Falls, across the river from Georgetown and just up river from Teddy Roosevelt Island. My house sits on top of a hill of hard rock, but at Teddy Roosevelt Island the softer sediments of the coastal plains begin.
The Fall Line extends roughly parallel to I-95, south past Fredericksburg, Virginia and Richmond and north up to Baltimore.
The Geology of the Fall Line has shaped every aspect of East Coast history, from how far upstream boats could navigate from the ocean, to where water mills could be constructed, and where native American tribes drew their pre-Columbian border lines.
Early colonists moving west beyond the Fall Line quickly discovered locations where river currents regularly scoured the bottom of streams flowing eastward across the Piedmont. Those rivers have a thin coating of recent sediments; mud, sand and stones deposited on the river bottom since the last major flood make crossing the rivers difficult in most locations. Colonists located the fords and later built their roads to take advantage of the places where hard bedrock was exposed underwater in the Piedmont. It was far easier to cross those rivers at a ford where bedrock was exposed, compared to dragging a wagon through mud.