Waterfalls form where a river flows over an area of hard rock followed by an area of softer rock.
The softer rock is eroded more than the hard rock, creating a "step" in the river.
As water goes over the step it erodes more and more of the softer rock.
A steep drop is eventually created, which is called a waterfall.
The hard rock is eventually undercut by erosion. It becomes unsupported and collapses.
The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion. This creates a deep plunge pool.
Over time, more undercutting causes more collapses. The waterfall will retreat (move back up the channel), leaving behind a steep-sided gorge.
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