There are many examples of the latter along the North and South Downs in southern England. Notably the National Trust-owned Devil's Dyke near Brighton covers some 200 acres (0.81 km2) of downland scarp, and includes the deepest dry valley in the world - created when melting water eroded the chalk downland to the permafrost layer after the last ice age. The three-quarter mile long curved dry valley is around 700 feet (210 m) in height and attracts tourists with its views of Sussex, Hampshire and Kent.
There are two theories as to how they were made, the first that the water table was once much higher, while the other is that during the ice age the area had tundra like conditions. The normally permeable chalk would have been made impervious by permafrost, thus allowing rivers to flow without carving into it. Today these valleys don't have any rivers because water sinks through into the limestone and flows underground in caverns.
There are many examples in the Peak District and the Yorkshire Wolds. A notable example is the valley of the River Manifold which is dry, except in spate, from Wetton south for several miles.
 See also
- "Hartley-Kent: Geology and Scenery of Hartley, Longfield and surrounding area". www.hartley-kent.org.uk. Retrieved 2010-08-09.
Media related to Dry valleys at Wikimedia Commons
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