🟥 SECTION 5: APPLYING YOUR KNOWLEDGE

A river flows over hard rock above softer rock. The softer rock is eroded, leaving a section of hard rock projecting outward. What feature has developed?

An overhang has developed.

The softer rock is less resistant to erosion, so it is worn away more quickly. The harder rock remains projecting outward.

A deep hollow develops at the base of a waterfall because of falling water and rock fragments. What is this feature called?

A plunge pool.

The force of falling water and the erosion caused by rock fragments deepen the hollow at the base of the waterfall.

A waterfall gradually moves backwards towards the source of a river. What process is occurring?

Waterfall retreat, associated with headward erosion.

Erosion and the collapse of rock cause the waterfall to move upstream.

A waterfall repeatedly erodes rock, undercuts harder rock and causes overhangs to collapse. What landform may develop behind it?

A steep-sided gorge.

Repeated erosion and collapse cause the waterfall to retreat upstream, leaving a deep, narrow valley behind.

Explain how differences in rock resistance can lead to waterfall retreat and gorge formation.

Hard rock is more resistant to erosion than soft rock. As water flows over the rock, the softer rock is eroded more quickly, creating a plunge pool and undercutting the harder rock above it.

The harder rock eventually forms an overhang and may collapse when it is no longer supported. This causes the waterfall to retreat upstream.

Repeated erosion and collapse gradually leave a steep-sided gorge behind the retreating waterfall.