dry valley

Valley without a river at its bottom. 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.