Describe the formation of a waterfall

Waterfalls form where the river bed has a layer of harder rock overlying the softer rocks. As water falls the softer rock is eroded more quickly than the harder rock leading to undercutting of the harder rock. The hard, overrhanging rock that is left eventurally collapses. The collapsed rocks fall into the plunge pool causing more erosion of the soft rocks and a deeper plunge pool. This goes on continuously causing the waterfall to retreat upstream. Furthermore, the waterfall leaves a steep sided gorge as it retreats.