The river flows over bands of less resistant (softer) and resistant (harder) rocks.
The less resistant rock is more quickly worn away due to differential erosion.
The river erodes the rocks in three main ways:
Hydraulic action – when the sheer force of the water gets into small cracks and breaks down the rock.
Corrasion – when the river bed and banks are eroded by the load hitting against them.
Corrosion – when the river water dissolves minerals from the rocks and washes them away.
The river undercuts the harder rock leaving an overhang which becomes unsupported and collapses into the plunge pool below.
After the overhang falls, some of the rocks are swirled around by the river and this helps to form a deep plunge pool below the waterfall. The plunge pool is also deepened during times of high discharge when hydraulic action is most powerful.
The waterfall is moved upstream, the process continues and a steep-sided gorge is cut back into the hillside.