fall line

The imaginary line along the eastern United States where the hard, resistant rock of the Piedmont meets the soft sediments of the Coastal Plain — marked by waterfalls and rapids where rivers drop off the harder rock onto the softer. The Fall Line determined where cities were built: Richmond, Washington, Philadelphia, Trenton, and other eastern cities sit at the head of navigation, where boats had to stop and cargo had to be portaged. In skiing, the fall line is the path a ball would take if rolled straight downhill — the line of steepest descent.