Ashdown Forest - Geology

Geology

The underlying geology of Ashdown Forest is mostly sandstone, predominantly the Lower Cretaceous Ashdown Formation. This forms a layer varying from 500 feet (150 m) to 700 feet (210 m) thick, consists of fine-grained, silty interbedded sandstones and siltstones with subordinate amounts of shale and mudstone. It is the oldest Cretaceous geological formation that crops out in the Weald.

The Ashdown Formation has been exposed by the erosion, over many millions of years, of a geological dome, the Weald-Artois Anticline, a process which has left the dome's oldest layers, the resistant sandstones that form its central east-west axis, as a high forest ridge that includes Ashdown, St. Leonard's, and Worth forests. This forest ridge, the most prominent part of the High Weald, is surrounded by successive concentric bands of younger sandstones and clays, and finally chalk. These form hills or vales depending on their relative resistance to erosion. Consequently, what the viewer sees when looking north or south across the Weald from the heights of Ashdown Forest is a series of successively younger geological formations. These include heavily wooded lowlands formed on Weald Clay, the high Greensand Ridge escarpment that rises prominently to the north, and, on the horizon, the chalk escarpments of the North Downs and South Downs (see diagram, right).

The Ashdown Formation is the lowest (oldest) layer of the Hastings Beds, which comprise (in sequence) the Ashdown Formation, Wadhurst Clay Formation, and Tunbridge Wells Sand Formation, and which are now thought to be predominantly fluvial flood-plain deposits. The Hastings Beds in turn represent the oldest part of the series of Cretaceous geological formations that make up the Weald-Artois Anticline, comprising (in sequence, from oldest to youngest) the Hastings Beds, Weald Clay, Lower Greensand, Gault, Upper Greensand, and Chalk. The anticline, which stretches from south-east England into northern France, and is breached by the English Channel, was created soon after the end of the Cretaceous period as a result of the Alpine orogeny. Ashdown Forest is itself situated on a local dome, the Crowborough Anticline.

Much of the iron ore that provided the raw material for the iron industry of Ashdown Forest was obtained from the Wadhurst Clay, which is sandwiched between the Ashdown Sands and Tunbridge Wells Sands (the latter encircles Ashdown Forest forming an extensive district of hilly, wooded countryside). Outcrops of Wadhurst Clay, which occurs as both nodules and in tabular masses, are distributed discontinuously in a horseshoe shape around Ashdown Forest, which has influenced the historical geography of iron-working around the forest.

Like the rest of the Weald, Ashdown lay beyond the southern limits of Quaternary ice sheets, but the whole area was subject at times to a severe periglacial environment that has contributed to its geology and shaped its landforms.