Editing Downland (section)

==Geology==
{{main|Chalk Group}}
The Chalk Group is a sequence of [[Upper Cretaceous]] [[limestone]]s. The dominant [[lithology]] is relatively soft porous white chalk with only poorly-defined bedding. The chalk is classified as a [[Folk classification#Folk's carbonate classification|biomicrite]],<ref name="Clayton_&_Matthews_1987">{{Cite book |last1=Clayton |first1=C.R.I. |title=Deformation of Sediments and Sedimentary Rocks |last2=Matthews |first2=M.C. |year=1987 |isbn=9780632017331 |editor-last=Jones |editor-first=M.E. |series=Geological Society, Special Publications |volume=29 |pages=55–62 |chapter=Deformation, diagenesis and the mechanical behaviour of chalk |editor-last2=Prescott |editor-first2=R.M.F. |editor-last3=Prescott |editor-first3=R.M.E.}}</ref> with microscopic [[coccolith]]s and other fine-grained fossil debris in a matrix of [[micrite]] mud. Small amounts of silica were also deposited, mainly from [[sponge spicule]]s, which moved during [[diagenesis]] and accumulated to form [[flint]]s. The Chalk Group either directly overlies the impermeable uppermost [[Lower Cretaceous]] [[Gault Clay]] or permeable [[Upper Greensand Formation]] above the Gault Clay.

Since its deposition, the chalk in southern England has been uplifted, [[Fault (geology)|faulted]], [[Fracture (geology)|fractured]] and [[fold (geology)|folded]] by the distant effects of the [[Alpine Orogeny]]. The fracturing has greatly increased the chalk's permeability, such that it is a major [[aquifer]].<ref name="Butler_etal_2012">{{Cite book |last1=Butler |first1=A.P. |title=Groundwater Resources Modelling: A Case Study from the UK |last2=Hughes |first2=A.G. |last3=Jackson |first3=C.R. |last4=Ireson |first4=A.M. |last5=Parker |first5=S.J. |last6=Wheatear |first6=H.S. |last7=Peach |first7=D.W. |year=2012 |isbn=9781862393448 |editor-last=Shepley |editor-first=M.G. |series=Geological Society, Special Publications |volume=364 |chapter=Advances in modelling groundwater behaviour in Chalk catchments |doi=10.1144/SP364.9 |s2cid=140713528 |editor-last2=Whiteman |editor-first2=M.I. |editor-last3=Hulme |editor-first3=P.J. |editor-last4=Grout |editor-first4=M.W.}}</ref> [[Sedimentary basin]]s formed by [[rift]]ing during the Triassic to Early Cretaceous were [[inversion (geology)|inverted]] during the Late [[Paleogene]] to [[Miocene]] leading to the formation of structures such as the [[Weald–Artois Anticline|Wealden Anticline]] and the [[Purbeck Monocline|Portland-Wight Monocline]].<ref name="Hopson">{{cite journal | url=http://nora.nerc.ac.uk/15996/1/Hopson_IoW_Geo_History_PGEOLA-D-11-00048R1.pdf | title=The geological history of the Isle of Wight: an overview of the 'diamond in Britain's geological crown' | author=Hopson P. | journal=Proceedings of the Geologists' Association | year=2011 | volume=122 | issue=5 | pages=745–763 | doi=10.1016/j.pgeola.2011.09.007| bibcode=2011PrGA..122..745H }}</ref> Later erosion has produced the characteristic ridges of the downland landscape. The landscape was further modified during the [[Quaternary]] period by the area's proximity to the southern edge of the ice sheets formed during the [[Last Glacial Period|last ice age]]. These [[periglacial]] effects included significant amounts of dissolution of the chalk and the modification of existing valleys due to a combination of frozen ground and [[snowmelt]].<ref name="Ballantyne_&_Harris_1994">{{Cite book |last1=Ballantyne |first1=C.K. |author-link=Colin Ballantyne |title=The Periglaciation of Great Britain |last2=Harris |first2=C. |publisher=Cambridge University Press |year=1994 |isbn=9780521310161 |pages=153–154}}</ref>