In the diagram, above, marker a is pointing to a which is formed by water eroding away soft rock around harder rock, creating a steep drop-off.

Waterfall. 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.