How is a waterfall formed in a river?

A waterfall forms in the upper course of a river. The water here flows over hard and soft rock. The soft rock is eroded more quickly than the hard rock. The river undercuts the hard rock leaving an overhang which forms a waterfall. Where the soft rock has been eroded, a plunge pool forms. As erosion continues the waterfall retreats upstream forming a gorge. It is important to draw sketches to visualise the process. Therefore, the process is easier to remember during an exam.