Waterfalls occur when rivers flow over areas with different rock types. Softer rock erodes faster than harder rock, creating a step over which water plunges as a waterfall. As erosion continues, water cuts away rock behind the waterfall, causing it to move back upstream and leave a gorge. Over time, this process eats away at the soft rock, undercutting the hard rock above and causing sections to collapse, further deepening the plunge pool.