Describe the formation of a waterfall (usually a question worth 4 marks in an A-Level paper)

The formation of a waterfall is predominately a result of geology. Throughout the long profile of a river horizontal bands of soft and hard rock are passed over. The harder rock is generally on top of the underlying softer rock and thus it is called the cap rock. This leads to a differing rate of erosion as the underlying soft rock is eroded much more easily and faster than the hard rock above it. Examples of this erosion include abrasion (where the rock is eroded by the stones that are carried in the load of the river from erosion further upstream, pounding against the rock) and hydraulic action where the force of the water causes air to be wedged into cracks and crevices and is subsequently released with explosive pressure often dislodging pieces of rocks which have been mechanically weathered. The soft rock will over time be gouged out to leave an overhang as the profile of the river is steepened. Eventually the overhanging rock will be unable to support its weight and will collapse into the plunge pool at the base of the waterfall and will provide further material for erosion. This will lead to the deepening of the waterfall.