Devil's Dyke

Sussex RIGS number: TQ21/47

Devil's Dyke Geological Conservation Review (GCR) Site
   Block: Quaternary of South-East England (Site code 791)
   Block: Karst (Site code 1111)

Devil's Dyke is in the South Downs National Park

Grid Reference: TQ260110

Owner:
National Trust

Site access:
Directions based on National Trust
Free parking at Summer Down Road; pay & display parking at Devil's Dyke. £2 all day, National Trust members and Blue Badge holders park free. Suitable for coaches.
Open access land.

Summary Description:

24/06/2010
from West Sussex RIGS Survey 2010

Interest Feature(s)

Bedrock:
Stratigraphy: Zig Zag Chalk Formation, Grey Chalk Subgroup, Holywell Nodular, New Pit and Lewes Nodular Chalk Formations, White Chalk Subgroup, Late Cretaceous in age.

Superficial Deposits:
Deposits on the valley floor are of research interest and could provide further evidence about the formation of Devil's Dyke

Geomorphology:
Devil's Dyke is an impressive dry valley incised into the chalk escarpment. Its upper part cuts through the lower formations of the White Chalk and the lower part cuts into the underlying Grey Chalk. Devil's Dyke is just over 1 km long, 400m wide from rim to rim, and up to 80m deep.
The upper valley has a V-shaped profile with unusually steep slopes and shows a right angle bend into the lower valley. Formation of the valley probably involved capture of a former dip slope stream by headward erosion of a scarp face stream.
During most of postglacial time up to the present the valley has probably been 'normally' dry and channel flow, with the potential to achieve erosion, has occurred only during relatively rare storm events, and possibly during even rarer snow melt events, when the rate of surface run-off exceeded soil/rock infiltration capacity. Its origin is still debated but the valley was probably incised mainly during glacial episodes by high snow-melt runoff over relatively impermeable ice-rich permafrost, combined with periglacial slope processes (gelifluction).

Historical background:
Devil's Dyke is probably the best known of all the chalk dry valleys and has been visited and studied by many geomorphologists. A variety of hypotheses have been proposed to explain its formation.

Site map - aerial photography 2007 - survey visit 24/06/2010

Click locations on map to view photos

Site access:
Directions based on National Trust
Free parking at Summer Down Road; pay & display parking at Devil's Dyke. £2 all day, National Trust members and Blue Badge holders park free. Suitable for coaches.
Open access land.

RIGS Designation Criteria:
Scientific Details and Site Description
Geomorphology: Devil's Dyke is a coombe incised into the north-facing scarp face of the South Downs. The valley is just over 1000m long and 400m wide at its rim and reaches 80 deep.
There is still great uncertainty concerning its geomorphological evolution but the unusual steepness of this dry valley is believed to have evolved under both interglacial and periglacial conditions. The early stages may have involved the capture and rejuvenation of a dip slope valley by a scarp-foot spring and its associated stream. At a later stage the valley would have evolved under periglacial conditions.
Stratigraphy: The valley cuts through most of the Lower and Middle Chalk and finishes in the Upper Greensand.
Historical Value: This site has attracted the attention of a large number of geomorphologists, and a variety of theories have been put forward to explain the unusually steep sides and peculiar right-angle bend.
Educational Value: This site is of great geomorphological importance both at a regional and a national level. There can be few karstic or periglacial geomorphologists in thiscountry who have not visited the Dyke. Dry valleys of this type are relatively rare in the English Chalk, with about a dozen examples south of the Thames, with Devil's Dyke as the largest. It is an important Quaternary landform and even though it has aroused considerable scientific interest over the years, its potential scientific significance is still very great. Suitable for visits by large groups of all ages and backgrounds.
Biological importance: This area on the scarp slope of the South Downs has three nationally uncommon habitats: south-east chalk grassland, juniper scrub and calcareous pedunculate oak-ash-beech woodland. It is rich in invertebrates, especially harvestmen and has some uncommon butterflies and moths.
Archaeology: There are also several interesting archaeological earthworks.

SSSI Designation Criteria (citation):
Beeding Hill to Newtimber Hill situated on the scarp slope of the South Downs is a site of both geological and biological importance. Three nationally uncommon habitats are represented: south-east chalk grassland, juniper scrub and calcareous pedunculate oak-ash-beech woodland. The site supports a rich community of invertebrates, especially harvestmen and has some uncommon butterflies and moths. A nationally uncommon plant also occurs. Devil's Dyke is the best known example of a dry chalk valley.

Geology: Devil's Dyke is the most famous and remarkable of all the chalk dry valleys and is frequently cited as the type example. It is the largest single coombe anywhere in the chalk karst of Britain and is of considerable geomorphological interest in providing a most spectacular example of Pleistocene erosion of chalk. Its exact origin is a source of debate, but it seems to have evolved under both periglacial and interglacial conditions. A series of deposits on the valley floor has considerable research potential and may throw further light on the formation of the feature. Devil's Dyke is a key site for periglacial and chalk geomorphology.

GCR Site Account:
Block: Quaternary of South-East England (Site code 791)
Not available

Block: Karst (Site code 1111)
Not available.


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