File:Eroded hillside near Oliver's Castle - geograph.org.uk - 870366.jpg

English: Eroded hillside near Oliver's Castle The escarpment here is at the edge of chalk downland. Chalk readily absorbs rainwater so surface water in the form of streams and pools is only seen where the ground is below the water table, there us very little run-off.

These gullies and ridges were probably formed by the water table breaking through to the surface and running down the hillside. The level of the water table has been reduced by extraction over the last hundred years or so. These gullies and ridges are easily seen from a distance and make a useful pointer to Oliver's Castle. My thanks to other contributors to Geograph for their help with these comments.

Nigel Mykura has supplied the following: This is from tiscali reference:

Such valleys are common on the dip slopes of chalk escarpments, and were probably formed by rivers. However, chalk is permeable (water passes through it) and so cannot retain surface water. Two popular theories have arisen to explain how this might have happened: 1) During the last ice age the chalk might have frozen and been rendered impermeable. During the summer thaw, water would then have flowed over the land, unable to sink into it, and river valleys would have been formed. When, after the ice age, the chalk thawed and became permeable again, rivers could no longer flow along the valleys and so these became dry. 2) At the end of the last ice age so much meltwater might have been created that the water table would be far higher than it is today. This would have enabled water to flow over the chalk surface without being absorbed, and create valleys. As the water table fell with time, however, water passed through the chalk once more and the valleys became dry. Good examples include Devil's Dyke, Fulking, England, and the Vale of the White Horse, Oxfordshire, England.