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. Chalk ...

, 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 In geology, bedrock is solid Rock (geology), rock that lies under loose material (regolith) within the crust (geology), crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface mater ...

. 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 (geometry), direction or geography. Etymology T ...

and

South Downs The South Downs are a range of chalk hills that extends for about across the south-eastern coastal counties of England from the Itchen valley of Hampshire in the west to Beachy Head, in the Eastbourne Downland Estate, East Sussex, in the east. ...

Southern England Southern England, or the South of England, also known as the South, is an area of England consisting of its southernmost part, with cultural, economic and political differences from the Midlands and the North. Officially, the area includes G ...

. Notably the National Trust-owned Devil's Dyke near

Brighton Brighton () is a seaside resort and one of the two main areas of the City of Brighton and Hove in the county of East Sussex, England. It is located south of London. Archaeological evidence of settlement in the area dates back to the Bronze A ...

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 gree ...

. 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 thumb The Alkham Valley is in the Kent Downs (the eastern part of the North Downs), an Area of Outstanding Natural Beauty, in South East Kent, England. The valley lies between Folkestone and Dover. The Valley is a dry valley A dry valley may de ...

near

Dover Dover () is a town and major ferry port in Kent, South East England. It faces France across the Strait of Dover, the narrowest part of the English Channel at from Cap Gris Nez in France. It lies south-east of Canterbury and east of Maidstone ...

, and the Hartley Bottom and Fawkham valleys near

Dartford Dartford is the principal town in the Borough of Dartford, Kent, England. It is located south-east of Central London and is situated adjacent to the London Borough of Bexley to its west. To its north, across the Thames estuary, is Thurrock in ...

in north Kent. There are many examples of limestone dry valleys in the

Peak District The Peak District is an upland area in England at the southern end of the Pennines. Mostly in Derbyshire Derbyshire ( ) is a ceremonial county in the East Midlands, England. It includes much of the Peak District National Park, the southe ...

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 The River Manifold is a river in Staffordshire, England. It is a tributary of the River Dove (which also flows through the Peak District, forming the boundary between Derbyshire and Staffordshire). The Manifold rises at Flash Head just south ...

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 Periglaciation (adjective: "periglacial", also referring to places at the edges of glacial areas) describes geomorphic processes that result from seasonal thawing of snow in areas of permafrost, the runoff from which refreezes in ice wedges and ot ...

conditions during which normally permeable bedrock would have been made impervious by

permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...

, thus allowing flowing water to erode it.

References

External links

Valleys {{topography-stub

See also

References

External links