Editing Downland (section)

==Hydrology==
Chalk deposits are generally very permeable, so the height of the [[water table]] in chalk hills rises in winter and falls in summer.<ref name="Butler_etal_2012"/> This leads to characteristic chalk downland features such as dry valleys or [[Combe|coombe]]s, and seasonally-flowing streams or [[Winterbourne (stream)|winterbournes]]. The practice of extracting water from this aquifer, in order to satisfy the increasing demand for water, may be putting some of these streams under stress.

In the valleys below the downs at the base of the chalk layer, greensand or gault clay comes to the surface and at the interface at the top of the gault a [[Spring line settlement|springline]] can occur where water emerges from the porous chalk or the underlying greensand.  Along this line, settlements and farms were often built, as on the higher land no water was available. This is demonstrated very clearly beneath the scarp of the [[White Horse Hills]], above the [[Vale of White Horse]]. In many chalk downland areas there is no surface water at all other than artificially created [[dewpond]]s.<ref name="Bettey_2002"/>