Rivers pass over a hard cap rock with softer rock below. As the river erodes the softer rock faster than the hard rock, it creates an overhang. The overhang eventually collapses, forming a steep-sided gorge and waterfall that retreats upstream through further erosion.

1. Waterfalls
These 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

2. High Force – R. Tees

3. 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.

4. Waterfalls
•As the waterfall
retreats upstream it
leaves behind a
narrow, steep sided
valley or GORGE
•Vertical &
headward erosion

6. 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.

7. 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.