How do waterfalls form?

Waterfalls commonly form where water rushes down steep hillsides and quickly erodes the rocks. The height and number of waterfalls along a stream or river depends upon the type of rocks that are being eroded by the water. Some types of rocks (shale, for example) wear away more easily than others (such as sandstone or limestone).

As the river or stream wears away the weak rocks, they travel across the surface of stronger rocks. These more resistant rocks become the capstones to waterfalls. The number and thickness of these stronger rock units in a vertical sequence of rocks controls how many water falls there are and how much vertical drop there is on each waterfall.

Taughannock (tu-GANN-ic) Falls, pictured above, is the highest single-drop waterfall (70.5 meters, or 215 feet) in the northeastern United States. There are two reasons for this:

1. The creek that flows over Taughannock Falls has to drop many hundreds of feet to reach the water level of Cayuga Lake, which it empties into.
2. There is a thick layer of weak shale rocks between the strong sandstones at the top and the stronger limestones.

The water easily wore away the shale and left one big drop between the capstones, as shown below.