Gulls, gull-caves and cambering in the southern Cotswold Hills, England

The study of fissures and caves formed by mass movement is important not only because they are a significant geohazard, but because such caves can be repositories for palaeogeographic information. In southern Britain, such open joints in solid strata are known as gulls. In the southern Cotswold Hills, a sequence of interbedded limestones and mudstones has been deeply dissected by the River Avon and its tributaries. Mass-movement and cambering has opened up narrow gulls and larger gull-caves to a far greater extent than had previously been realized. Surveys made inside the extensive abandoned Box Freestone Mine have allowed the spatial distribution of these features to be mapped. We found that the direction of extension is not always in the direction of topographic gradient; locally it can be influenced by the stratal dip direction. In some areas, abrupt changes in the orientation of valley sides have allowed mass movement in two directions, creating rectilinear networks of gull-caves. A quantitative assessment of the valley-ward extension of the strata caused by gulling was made in Box stone mines. Extension may often exceed 5%, and locally may reach 10% in discrete zones. Evidence of preferential dissolution along the NW-SE joint set in these gull-caves suggests former groundwater flow to the northwest, and not to the present River Avon valley. This indicates that the River Avon has subsequently captured the dip slope streams that fed the River Thames headstreams. The greatly enhanced flow of the River Avon after this capture caused rapid over-deepening of the valley, which triggered the original cambering and thus formation of the gull-caves themselves.