waterfalls and gorges

• 1. waterfalls form where a river flows over an area of hard rock followed by an area of softer rock
• 2. the softer rock is eroded (by hydraulic action and abrasion)more than the hard rock, creating a 'step' in the river
• 3. as water flows over the step it erodes more and more of the softer rock
• 4. a steep drop is eventually created, which is called a waterfall
• 5. the hard rock is eventually undercut by erosion. it becomes unsupported and collapses
• 6. 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
• 7. over time, more undercutting causes more collapses. the waterfall retreats, leaving behind a steep-sided gorge