A river flows over an area where hard rock lies on top of soft rock. The soft rock is less resistant to erosion., Step 1, The river erodes the soft rock mainly through hydraulic action, where the force of the water forces air into cracks and weakens the rock., Step 2, Because the soft rock erodes faster, it is worn away beneath the hard rock. This creates undercutting, leaving the hard rock overhanging, Step 3, Falling water and rock fragments hit the base of the waterfall. Hydraulic action and abrasion deepen a hollow called a plunge pool., Step 4, The unsupported hard rock eventually collapses into the plunge pool. Over time, this causes the waterfall to retreat upstream., Step 5, As the waterfall continues to retreat, it leaves behind a steep-sided gorge., Step 6

Waterfall formation steps

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