A dry valley may develop on many kinds of permeable rock, such as

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...

, or sandy terrains that do not regularly sustain surface water flow. Such valleys do not hold surface water because it sinks into the permeable bedrock. There are many examples of chalk dry valleys along the

North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north ...

and South Downs in Southern England. Notably the National Trust-owned Devil's Dyke near Brighton covers some 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 An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...

. The three-quarter mile long curved dry valley is around in height and attracts tourists with its views of Sussex, Hampshire and Kent. Other examples include the Alkham Valley near Dover, and the Hartley Bottom and Fawkham valleys near Dartford in north Kent. There are many examples of limestone dry valleys in the Peak District and the

Yorkshire Wolds The Yorkshire Wolds are low hills in the counties of the East Riding of Yorkshire and North Yorkshire in north-eastern England. The name also applies to the district in which the hills lie. On the western edge, the Wolds rise to an escarpment wh ...

. A notable example is the valley of the River Manifold which is dry, except in spate, from Wetton south for several miles. Many hypotheses have been advanced to explain dry valley development. Some may have developed during periods of higher

water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...

, others in periglacial conditions during which normally permeable bedrock would have been made impervious by permafrost, thus allowing flowing water to erode it.

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Valleys {{topography-stub

See also

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