Describe the formation of a waterfall

Waterfalls are formed over areas where hard rock lays over soft rock. When the river flows over soft rock, it erodes the soft rock away, which begins to undercut the hard rock which lays above. As the hard rock falls, it drops into a plunge pool below, which gets deeper and wider by the process of abrasion (the fallen rock erodes away the sides of the plunge pool). This process continues, demonstrating the retreat of the waterfall upstream as hard rock is continually undercut, becoming unstable, and falls into the pool below.