Rivers and coasts, Year 8: more practice

Hydraulic action

提示The sheer force of the water itself does the work.

为什么Each 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

提示Compare how quickly different rocks wear away.

为什么Waves 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

提示The one that is easier to erode.

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

A gorge

提示Think of a narrow, deep valley with rocky walls.

为什么Each time the overhang collapses the waterfall moves back, and the channel it abandons becomes a gorge.

It deposits sediment, building a slip-off slope

提示Slow water cannot carry as much load.

为什么The 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

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

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

An arch

提示Think of a rock doorway into the sea.

为什么Waves keep eroding the back of the cave until it breaks through the other side of the headland, leaving an arch.

A stump

提示Think of what is left when a tree is cut down.

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

Impermeable rock

提示Think about the land the rain falls on.

为什么On 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

提示Think about what leaves and roots do with rainwater.

为什么Trees 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.