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 erosionErosion is the process that wears away the river bed and banks. Erosion also breaks up the rocks that are carried by the river..
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 poolThe pool of water found at the bottom of a waterfall. It is an erosional feature which has been created by a combination of hydraulic action and the abrasion of the plunging water. 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 dischargeThe volume of water in a river passing a point in a given time. Measured in cumecs (cubic metres per second). 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.