Explain the formation of a waterfall

The is a layer of harder rock on top of soft rock. The softer rock underneath erodes more quickly, undercutting the hard rock. This leave the hard rock to over hang. Eventually the soft rock is undercut so much the hard rock overhang collapses. This is the initial formation of the waterfall. When the hard overhang collapses into the plunge pool, more erosion occurs. This causes the pool and therefore the waterfall to deepen. Each time the overhang collapses, the waterfall retreats leaving a GORGE. This process is continuous.