Middle/Upper Jurassic (Kellaways Beds and Oxford Clay), Bristol and Gloucester region
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
The Kellaways Beds, named after the hamlet of Kellaways in Wiltshire, have been mapped between Meysey Hampton, 6 km north of Cricklade, and Trowbridge, They are subdivided into the Kellaways Clay and the Kellaways Sand, which forms a discontinuous mappable stratum at the top. The Kellaways Clay consists mainly of silty clay with impersistent thin layers and lenticular beds of sand. It is grey when fresh and at outcrop weathers orange-brown. The Kellaways Sand, where unweathered, is a pale grey, hard calcareous sandstone or sandy fossiliferous limestone (sometimes known as Kellaways Rock), but at outcrop it is a gritty sand. It gives rise to a light sandy loam soil, easily cultivated, and commonly supporting market gardening. The thickness of Kellaways Beds varies from 20 to 30 m, of which the sand commonly accounts for only 2 to 3 m, though it may attain as much as 9 m north-west of Chippenham. Boreholes at Tytherton have provided zonal details of the lower beds. In the southern part of the district the Kellaways Beds have not been separately mapped from the Oxford Clay, though their presence is indicated by lighter soil and more undulating topography, together with evidence from temporary exposures and boreholes.
The youngest Jurassic formation represented in the region is the Oxford Clay, which encompasses most of the Callovian Stage and the greater part of the Lower Oxfordian. The base of the Upper Jurassic is taken at the base of the Oxfordian Stage. Although the formation is on average about 150 m thick it is poorly exposed in the present district. It generally gives rise to heavy clay with a faintly undulating surface channelled by small streams. The Oxford Clay is mainly tough, smooth, greenish grey clay except in the lower part where it is appreciably coarser grained and passes down into the Kellaways Beds.