Rivers usually flow over a mixture of hard and soft rock. The force of the water will wear away more soft rock than harder rock. This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.
A fall line is the imaginary line along which parallel rivers plunge as they flow from uplands to lowlands. Many waterfalls in an area help geologists and hydrologists determine a region’s fall line and underlying rock structure.
As a stream flows, it carries sediment. The sediment can be microscopic silt, pebbles, or even boulders. Sediment can erode stream beds made of soft rock, such as sandstone or limestone. Eventually, the stream’s channel cuts so deep into the stream bed that only a harder rock, such as granite, remains. Waterfalls develop as these granite formations form cliffs and ledges.
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