The river cuts down into the valley. If there are areas of hard rock which are harder to erode, the river will bend around it. This creates interlocking spurs of land which link together like the teeth of a zip.
How does interlocking spur form?
As the river erodes the landscape in the upper course, it winds and bends to avoid areas of hard rock. This creates interlocking spurs, which look a bit like the interlocking parts of a zip. When a river runs over alternating layers of hard and soft rock, rapids and waterfalls may form.
What are cut off interlocking spurs called?
U-shaped valleys ending with a waterfall at the cliff-face are called hanging valleys. When a river erodes the landscape, ridges of land form in its upper course which jut into the river. These are called interlocking spurs. A glacier cuts through these ridges leaving behind truncated spurs.