Caves of the Mendip Hills
The caves of the Mendip Hills are formed by the particular geology of the Mendip Hills: large areas of limestone worn away by water makes it a national centre for caving. The hills conceal the largest underground river system in Britain.
The hills consist of anticlines of Carboniferous Limestone lying over Devonian Old Red Sandstone, with the sandstone exposed on the summits. When a surface stream running off the sandstone reaches the limestone it sinks below ground through a "swallet", (also known locally as a "slocker"), continuing on its way down towards sea level by enlarging existing cracks in the rock to form caves, and reappearing at the base of the limestone outcrop. As the water changes route within the hill some caves (or parts of caves) are left dry.
There is a characteristic type of Mendip cave, in which there is an initially steep descent, and then a more level stretch ending in a "sump" as the cave reaches and descends below the prevailing water table. The passages below the water table (which may be accessible by cave diving) often have a loop formation caused by the water flowing down a bedding plane and then rising up a fracture in the rock. Few caves on the Mendip Hills are more than 150 metres (492 ft) deep or a few kilometres in length, (with some notable exceptions, such as Swildon's Hole which is over 9 kilometres (6 mi) long), but a number are complex with several levels of passages which have been abandoned by the water, for example those around Cheddar.
The catchment area of the Cheddar Yeo, which rises in Gough's Cave, measures 54 km2 (20.8 sq mi). Dye marking shows that some of the water travels underground for up to 10 miles (16 km), taking up to 14 days to reach Cheddar.
When Banwell Bone Cave was discovered in 1824, it was found to contain a great many animal bones which have been dated as about 80,000 years old.
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