Rivers and coasts, Year 8: more practice

Hydraulic action

HintThe sheer force of the water itself does the work.

The whyEach wave pushes air in the crack under pressure, and as the water drains back the air expands. Repeated again and again, this breaks rock apart.

The soft rock erodes faster, leaving the hard rock behind

HintCompare how quickly different rocks wear away.

The whyWaves cut back soft rock such as clay more quickly than hard rock such as chalk or granite. This is called differential erosion.

The soft rock

HintThe one that is easier to erode.

The whyThe soft rock below is undercut, leaving the hard rock as an overhang. Eventually the overhang collapses and the waterfall steps back upstream.

A gorge

HintThink of a narrow, deep valley with rocky walls.

The whyEach time the overhang collapses the waterfall moves back, and the channel it abandons becomes a gorge.

It deposits sediment, building a slip-off slope

HintSlow water cannot carry as much load.

The whyThe sediment piles up into a gently sloping bank of sand and gravel, while the fast outside bend erodes a steep river cliff.

An oxbow lake

HintIt is named after a curved part of an old farm yoke.

The whyDeposition seals the ends of the abandoned loop, leaving a curved lake. Over time it can dry out into marshy ground.

An arch

HintThink of a rock doorway into the sea.

The whyWaves keep eroding the back of the cave until it breaks through the other side of the headland, leaving an arch.

A stump

HintThink of what is left when a tree is cut down.

The whyThe sequence is crack, cave, arch, stack, stump. Waves keep eroding the stack until only a low remnant remains.

Impermeable rock

HintThink about the land the rain falls on.

The whyOn impermeable rock, water cannot sink in, so it runs over the surface to the river and the river fills faster.

Fewer trees intercept rain and take up water, so more runs quickly to the river

HintThink about what leaves and roots do with rainwater.

The whyTrees catch rain on their leaves and draw water up through their roots. Without them, water reaches the river faster and the peak flow is higher.