Softer rock on the river bed gets eroded quicker than hard rock
River erodes the softer rock vertically creating an overhang of the hard rock
A plunge pool is created at the bottom of the overhang due to hydraulic action and corrasion.
The overhang collapses while the river keeps on eroding the softer rock below it.
The waterfall continues to retreat up stream due to successive overhang collapses.
A gorge is formed where the waterfall once was