A
fall line (or
fall zone) is the
geomorphologic break between an upland region of relatively hard
crystalline basement rock and a
coastal plain of softer
sedimentary rock. A fall line is typically prominent when crossed by a river, for there will often be rapids or
waterfalls. Many times a fall line will recede upstream as the river cuts out the uphill dense material, often forming “c”-shaped waterfalls. Because of these features riverboats typically cannot travel any farther inland without
portaging, unless locks are built there. On the other hand, the rapid change in elevation of the water, and the resulting energy release, makes the fall line a good location for
water mills,
grist mills, and
sawmills. Because of the need for a river port leading to the ocean, and a ready supply of water power, settlements often develop where rivers cross a fall line.