Waterfalls occur when a band of more resistant rock (e.g. granite) overlies a band of less resistant rock (e.g. sandstone)
- As the river passes over the resistant rock, the soft rock below is eroded underneath. This leaves the hard rock elevated above the stream below.
- The drop gets steeper as the river vertically erodes the soft rock through abrasion and hydraulic action. A plunge pool forms at the base of the waterfall
Eventually the hard cap rock collapses, as it is unsupported. The rock falls into the plunge pool and will continue to enlarge it due to abrasion,
A steep sided gorge is left behind as the processes of undercutting and collapse continue