WaterfallsThese are typical of upland rivers.

Here the river passes over a hard CAP rock.

Below the cap rock is a softer, more easily erodable rock

High Force, R. Tees

High Force – R. Tees

Waterfalls •The river can not easily erode the hard cap rock

•The softer underlying rock is eroded, creating an overhang

•The overhang collapses & the waterfall retreats upstream

•Hydraulic action & abrasion scour out a deep plunge pool, helped by the overhang collapse.

Waterfalls •As the waterfall retreats upstream it leaves behind a narrow, steep sided valley or GORGE

•Vertical & headward erosion

Waterfall formation WAGOLL

Water flows over hard rock then soft rock. As the water flows over the soft rock, it erodes faster than the hard rock through the processes of hydraulic action which is the sheer force of the water wearing away the bed of the river making it steeper. Eventually this causes a vertical drop in the river with a deep pool called a plunge pool at the base. As the water flows over into the plunge pool it undercuts the hard rock by eroding the soft rock.The undercut, now forms an overhang of hard rock which will eventually collapse into the plunge pool. This process repeats itself faster due to abrasion, which is where rocks in the river erode the bed, causing the waterfall to retreat upstream forming a steep sided gorge.