Waterfalls 

A waterfall is a steep drop in the course of a river.

They form when a band of hard resistant rock (cap rock) lies over softer, less resistant rock.

The softer rock is quickly eroded by hydraulic action and abrasion, causing the harder rock to be undercut.

The hard rock overhangs until it can no longer carry its own weight.

The overhang collapses and then breaks up in the water below.

The great power of the water at the base of the waterfall causes a plunge pool to form.

The bed of the river below the waterfall contains boulders eroded by splash back from behind the waterfall.

Gorges 

Over time the process above is frequently repeated and eventually a steep-sided gorge forms as the waterfall retreats up stream.