which finishes at a cascade falling from a long lake. There are also more than of high-level passages above the river. The way on continues underwater for some reaching a depth of before surfacing in the 25th chamber – called the Lake of Gloom because of its thick mud deposits. The sump at the end of this has been dived for to a maximum depth of before gravel chokes prevented further progress. The end is about northeast of the entrance. Hydrology and geology Wookey Hole is on the southern escarpment of the Mendip Hills, and is the resurgence that drains the southern flanks of North Hill and Pen Hill. It is the second-largest resurgence on Mendip, with an estimated catchment area of , and an average discharge of per second. Some of the water is allogenic in origin i.e. drained off non-limestone rocks, collecting as streams on the surface before sinking at or near the Lower Limestone Shale — Black Rock Limestone boundary, often through swallets such as Plantation Swallet near St Cuthbert's lead works between the Hunter's Lodge Inn and Priddy Pools. It then passes through major cave systems such as Swildon's Hole, Eastwater Cavern and St Cuthbert's Swallet, around Priddy, but 95% is autogenic water that has percolated directly into the limestone. The southern slopes of the Mendip Hills largely follow the flanks of an anticline, a fold in the rock that is convex upwards and has its oldest beds at its core. On the Mendips the crest of the anticline is truncated by erosion, forming a plateau. The rock strata here dip 10–15 degrees to the southwest. The outer slopes are mainly of Carboniferous Limestone, with Devonian age Old Red Sandstone exposed as an inlier at the centre. Wookey Hole is a solutional cave, mainly formed in the limestone by chemical weathering whereby naturally acidic groundwater dissolves the carbonate rocks, but it is unique in that the first part of the cave is formed in Triassic Dolomitic Conglomerate, a well-cemented fossil limestone scree representing the infill of a Triassic valley. The cave was formed under phreatic conditions i.e. below the local water table, but lowering base levels to which the subterranean drainage was flowing resulted in some passages being abandoned by the river, and there is evidence of a number of abandoned resurgences. In particular, the passages in the 20th chamber are interpreted as a former Vauclusian