An account is given of the exploration of Charterhouse Cave, including early work by this Society, more recent excavations by the Sidcot School Speleological Society and the final opening in 1981. The accessible passages are briefly described. The cave contains a typical Mendip cave fauna, with a well developed twilight community and a troglobite fauna comprising two Crustacea and a springtail. The population of the troglophile, Speolepta leptogaster, will probably be most sensitive to the opening of the system. The cave is developed in a structurally complex monoclinal flexure, with strong jointing perpendicular to the east-west fold axis. Phreatic remnants are present as high as 240 m AOD, but the local water-table elevation declined gradually with time to below the present end of the system. A sequence of six vadose trenches adjusted to temporary still-stands of the local water-table are proposed and evidence of nine sometimes spatially discrete fill phases cited to support this contention. These fills also assist in the recognition of a minimum of five separate significant inlets to the system, which have varied in importance through time. Down-dip modification of the existing mature phreatic system is stressed. This evolution scheme is contrasted with that of Ford (1964) for the nearby G.B. Cave, where it may also apply.

Between 1844 and 1858 the ‘adventurers’ of the Mendip Hills Mining Company sank 6 deep shafts at Charterhouse in search of lead ore, without success. From 1847 to 1885 they reworked almost the whole deposit of lead-rich refuse that had been left by earlier miners in Blackmoor and Velvet Bottom valleys. The venture was beset by disasters and lost money until Nicholas Ennor was employed as a consultant. In 1861 control passed to a large Cornish smelting firm, Treffry’s and thereafter profits were made. Detailed accounts are given of the existing remains of mining and slagging.

The morphology and deposits of Rhino Rift are described. The cave is an ‘invasion vadose cave’ formed when local saturation level was 75-90 m OD. Ten U-series dates on four speleothems show that a boulder and cobble infill at the bottom of the known cave accumulated sometime after c. 45,000 years ago, whereas poorly sorted muddy gravels blocking the entrance passage were laid down before c. 11,000 years ago. The cave itself was formed before 75,000 years ago, probably during Isotope Stages 5 or 6. The implications for the geomorphic history of the Mendip region are discussed.

A bedrock depression revealed during construction work, together with its sedimentary infill, is described. It is concluded that the depression is probably a small fossil swallet cave system, infilled with sediments which are fluvial at the base, overlain by solifluction and loessic head. In a discussion of the wider regional geomorphology it is suggested that the site described was on an early high level valley route from the Old Red Sandstone in the north, towards the Severn-Wye system. This route had been substantially dissected by river captures and the drainage has now been captured by the Otter Hole cave system.