South Downs National Park - Geology

Geology

Most of the national park consists of chalk downland, although a significant part includes the sandstones and clays of the western Weald, a strongly contrasting and distinctive landscape of densely-wooded hills and vales.

The chalk was formed in the Late Cretaceous epoch, between 100 million and 65 million years ago, when the area was under the sea. During the Cenozoic era the chalk was uplifted as part of the Weald uplift which created the great Weald-Artois Anticline, caused by the same orogenic movements that created the Alps. The relatively resistant chalk rock has, through weathering, resulted in a classic cuesta landform, with a northward-facing chalk escarpment that rises dramatically above the low-lying vales of the Low Weald.

The chalk escarpment reaches the English Channel west of Eastbourne, where it forms the dramatic white cliffs of Beachy Head, the Seven Sisters and Seaford Head. These cliffs were formed after the end of the last Ice Age, when sea levels rose and the English Channel was formed, resulting in under-cutting of the chalk by the sea.

The South Downs run linearly west-north-westwards from the Eastbourne area through southern Sussex to the Hampshire downs, separating the south coastal plain from the clays and sandstones of the Weald. Behind the escarpment, on the dip slope, are the characteristic high, smooth, rolling downland hills interrupted by dry valleys and wind gaps, and the major river gaps of the Cuckmere, Ouse, Adur and Arun.

The chalk is a white sedimentary rock, notably homogeneous and fine-grained, and very permeable. It consists of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. The strata include numerous layers of flint nodules, which have been widely exploited as a material for manufacture of stone tools as well as a building material for dwellings. Similar areas in Britain include the North Downs and the Chilterns.

In its western section, the national park extends north beyond the chalk escarpment of the South Downs into a quite different and strongly contrasting physiographic region, the western Weald, taking in the valley of the western River Rother, incised into Lower Greensand bedrock, and the densely wooded hills and valleys of the Greensand Ridge and Weald Clay south of Haslemere.