Wealden district - The Weald
The Weald is a broad area of ridges, separated by clay valleys of various widths. It occupies the southern parts of Kent and Surrey, and the northern parts of Sussex, extending into the eastern fringe of Hampshire. The area is underlain by varied deposits that originated as sediments on the ancient sea-floor, or in low-lying coastal areas, and which now form successive layers beneath the Chalk. The parts of the younger sedimentary bedrock that can be seen at the surface in the Weald largely comprise mudstone (Gault Clay, and Weald Clay of the Low Weald), alternating with sandstones or with mixed sequences of sandstone, siltstone and mudstone (Upper Greensand, Lower Greensand and the Wealden Group of the High Weald). The individual layers vary from a few centimetres to tens of metres in thickness. Some of the younger sandstones are notably lime-rich, giving rise to local building stones such as ‘Kentish ragstone’ (Plate P209590). Some of these intervening layers also provide aquifers of local importance. The water in these aquifers either flows between the rock grains or through cracks in the rock, and sometimes both.
The mudstone intervals can be as little as a few metres thick. The thickest, known as the Weald Clay (Plate P209934), which forms the clay lands of the Low Weald, is up to 460 m thick in the west of the area but thins to as little as 122 m in the east. It was deposited in a broad shallow lake or coastal lagoon, with meandering rivers.
Deeper levels of the younger sedimentary bedrock include some thick limestones (Portland, Purbeck, Corallian, Great and Inferior Oolite groups). These alternate with further thick mudstone sequences (Kimmeridge, Ampthill and Oxford clays, and the Lias and Mercia Mudstone groups) and some sandstones (in the Wealden Group and the Sherwood Sandstone Group) ; the complete sequence reaches 2 km thick in the centre of the Weald. The thicker limestones and some of the sandstones are more porous and therefore act as reservoirs for the region’s oil and gas resources, while the dense clays act as barriers to the flow of oil, gas and water.
Beneath the younger sedimentary bedrock lie older sedimentary rocks (Carboniferous and Devonian rocks) that have experienced considerable folding and faulting over a long period culminating in a period of mountain building about 300 million years ago. Rocks representing this same zone may be seen in the cliffs of Devon and Cornwall, but in the Weald they usually occur at depths greater than 1 km.