system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't waterfalls get worn down to gradual inclines?
gpt
Waterfalls form where a layer of hard rock lies on top of a layer of soft rock. The soft rock below erodes faster, creating a steep drop, sheltered by a ledge of hard rock above. Eventually, the soft rock starts to erode beneath the hard rock, creating an overhang, which breaks off when it gets too long, creating a new vertical face, so the waterfall gradually moves upstream.