SPELEOGENESIS OF THE MOUNT ELGON 'ELEPHANT CAVES', KENYA

The eastern flanks of Mt. Elgon, a ~4000 km², 4321 m early Miocene stratovolcano, are penetrated, at altitudes of 2400-2500 m, by several voluminous caves up to ~160 m long, ~80 m wide, and 20 m high. These caves are regularly visited by a variety of animals including elephants which 'mine' the pyroclastic bedrock for its content of sodium-calcium-rich salts. Since the first written descriptions in the late 19^th Century, their origin has been debated. Some authors have argued for primarily zoogeomorphic speleogenesis, whilst others have argued for primarily dissolutional origin with only secondary modification by elephants. We report the first serious mapping and geomorphological study of the caves. Cave development is strongly related to lithology. Percolation is limited to a lower ~1-2 m swelling-clay tuff and upper ~0-30 cm baked paleosol. These are separated and overlain by pyroclastic agglomerates of varying mineralogy and hardness. Caves develop behind waterfalls under surface stream valleys, by groundwater sapping, usually of the highly incompetent clay. This is followed by collapse of overlying agglomerate layers. The dominant passage shape is breakdown dome, with abundant fresh collapse blocks. Geophagy by elephants and other species, and some human mining for salts, significantly modify and enlarge the caves. These activities, focused on accessible and salt-rich units, create sizeable quasi-horizontal undercuts (up to ~3 m tall and deep), the locus moving upwards as collapse raises the floor. Significant erosion also occurs by biogeochemical activity from huge bat colonies. Pressure release jointing, efflorescence and flaking also occur. This ongoing removal of material allows continued collapse, while lateral extension continues as the clay unit fails. Quaternary glaciation and deglaciation may have modified the balance of speleogenetic processes by alternately reducing animal activity and increasing water flow. No evidence was found of phreatic or vadose activity, although round tree-trunk moulds superficially resemble phreatic tubes, while elephant-worn pathways may resemble vadose channels.