By rivers, Northumberland

Rivers in highland areas are very erosive and cut V-shaped valleys. This is because the rock material which they carry wears away the stream bed and banks, a process known as attrition.

Sand and gravel particles bounce along the river-bed, whilst pebbles and even boulders (after heavy rain) are rolled or dragged, wearing away the rock beneath.

If there is a sudden change from hard to softer rock, waterfalls often form. Here the river Tees is flowing over the edge of a hard basalt sill (the same one as at Hadrian’s Wall) onto the softer rocks beneath. At the foot of the fall, pebbles and boulders brought down the waterfall help to erode a deep plunge pool.