Exam Answer: The waterfall flows over resistant hard/cap rock. A deep plunge pool is formed at the base of the waterfall due to erosion. The hard rock above is undercut by erosion of the underlying soft rock. Eventually the overhang collapses. This collapse causes the waterfall to retreat upstream leaving steep cliffs instead of river banks. A gorge of recession is formed eg. Cheddar Gorge.
The answer to this question would be explained using diagrams, labels and pictures of real life examples (and if available, cake!). Exam technique would be taught by breaking down the process into six key points in order to ensure that the question is fully answered and achieves maximum marks.