Two gull caves from the Wiltshire/Avon border (UBSS Proceedings v.17(2))

Abstract: Descriptions and surveys of two caves in the Great Oolite Limestone east of Bath are presented. Henry's Hole is a mass movement cave, a gull structure, which runs into part of an old freestone mine. The mined workings allow access to other gulls which are not open to the surface. Although the surface strata appear mainly intact, the beds at depth are foundered in a complex manner which suggests that the gulls seen are part of a high level secondary disturbance, with major gulling (unseen) at greater depth. Sally's Rift is an entirely natural network of gulls, formed by the opening of two sets of joints. The cave has been known for many years but the only previously published survey is an incomplete Grade 1 sketch. Sally's Rift shows a most unusual feature for a gull structure, a bedding cavity which is thought to be caused by block subsidence between vertical and inclined joints. Two phases of mass movement can clearly be demonstrated by the presence of a collapsed false-floor in the cave. To help distinguish between caves formed by mass movement and those formed by solution, the characteristic features of gull caves are described. A morphological classification of gulls is also offered, based on and extending a system devised by Hawkins and Privett (1981). The theory that gull formation is caused by gravitational sliding on cambered hillsides is discussed and generally endorsed.