Explain how a waterfall forms with reference to the processes involved

The formation of a waterfall begins when geological differences occur in the rock over which a river flows. The most common set up is a layer of more resistant 'cap rock' lying on top of a larger layer of less resistant rock. As the water flows over the more resistant rock layer and down it will slowly erode the less resistant layer through two main processes: abrasion and hydrualic action. Abrasion occurs as rocks and other matter transported within the flow are thrown against the less resistant layer causing damage, and hydraulic action is the force of the water going over the cap eroding the less resistant layer. Overtime the gouging out of the less resistant rock will create an unstable overhang of the cap rock which will eventually break off. The material remains in the plunge pool for a time and is used to continue the process of abrasion, contributing to further erosion of the less resistant layer. The repeat of this process is known as waterfall retreat and leaves a gorge in its wake. I would always reccomend a time series diagram for this question.