How do waterfalls form?

Many waterfalls form where a river crosses from hard, resistant rock onto softer rock. The softer rock wears away faster, creating a step in the riverbed that grows into a vertical drop. As water plunges over the edge, it erodes the softer rock at the base, sometimes carving a deep plunge pool and undercutting the hard rock above. Over time this causes the waterfall to slowly retreat upstream, as at Niagara Falls, which has moved back over a long distance. Waterfalls can also form from glacial action, faulting, or where rivers reach a cliff at the coast.