Explain the formation of a waterfall (Physical Geography)

Waterfalls tend to form where a river bed has a layer of hard rock on top of softer rock. When this occurs the process outlined below may happen
1. Water falls. The softer rock is eroded more quickly than harder rock. Leads to undercutting of the harder rock.2. The hard, and now overhanging rock eventually collapses as the softer rock no longer exists beneath it.3. Collapsed rocks fall into the plunge pool below. These rocks cause more erosion of the soft rocks and deepen the plunge pool.4. 1-3 repeats continuously, causing the waterfall to retreat upstream.5. The retreating of the waterfall may lead to the formation of a steep sided gorge