Drainage-line silcretes of the Middle Kalahari: an analogue for Cenozoic sarsen trains?

This paper describes models put forward to explain the development of silcr etes within drainage lines at the distal end of the Okavango Delta system i n the Middle Kalahari of Botswana, and proposes that they provide an analog ue for the formation of sarsen stones within sarsen trains. The models desc ribe silcrete formation in shallow pans situated within liver valleys, with silicification resulting from the accumulation of fluvial inputs of elasti c material, silica from groundwater and additions of silica phytoliths from aquatic vegetation in seasonal pools. It is suggested, on the basis of mac ro- and micromorphological comparisons, that sarsens in the Clatford Bottom area of Wiltshire formed by this mechanism. The sarsens would have origina lly formed a spatially-limited linear silcrete body and would have then acc umulated within contemporary valleys during the course of landscape evoluti on. The implications of this model for the environmental conditions associa ted with sarsen formation and the likely timing of silicification are subse quently discussed. It is concluded that the geomorphological setting of sar sen formation may have been more important than climatic conditions at the time(s) of silicification.