Geology
The South Downs are formed from a thick band of chalk which was deposited during the Cretaceous Period around sixty million years ago within a shallow sea which extended across much of northwest Europe. The rock is composed of the microscopic skeletons of plankton which lived in the sea, hence its colour. The chalk has many fossils, and bands of flint occur throughout the formation. The Chalk is divided into the Lower, Middle and Upper Chalk, a thin band of cream-coloured nodular chalk known as the Melbourn Rock marking the boundary between the Lower and Middle units.
The strata of southeast England, including the Chalk, were gently folded during a phase of the Alpine Orogeny to produce the Weald-Artois Anticline, a dome-like structure with a long east-west axis. Erosion has removed the central part of the dome, leaving the north-facing escarpment of the South Downs along its southern margin with the south-facing chalk escarpment of the North Downs as its counterpart on the northern side, as shown on the diagram. Between these two escarpments the anticline has been subject to differential erosion so that geologically distinct areas of hills and vales lie in roughly concentric circles towards the centre; these comprise the Greensand Ridge, most prominent on the north side of the Weald, where it includes Leith Hill, the highest hill in south-east England, the low-lying clay vales of the Low Weald, formed of less resistant Weald Clay, and finally the more highly resistant sandstones of the High Weald at the centre of the anticline, whose elevated forest ridge includes most notably Ashdown Forest.
The chalk, being porous, allows water to soak through; as a result there are many winterbournes along the northern edge.
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