Sunday, 7 August 2011
Wiltshire Geology Group: "Scattered over the Marlborough Downs and Kennet valley, where the Tertiary sediments originally covering the Chalk have been eroded away, are areas of sarsen stones. These lumps of hard sandstone lying at or just below the surface are silcretes, the result of localised patchy cementation of Tertiary sands. The ubiquitous presence of fossil root holes in the sarsens, indicates that this cementation occurred at the same time that the plants were growing, near the ground surface. If the sands were situated around the level of the top of the water-table, in the warm climate prevailing during the Tertiary period water would be drawn up through the sands by capillary action as evaporation occurred at the surface; this would concentrate dissolved silica in the ground water to the point where locally it crystallised out, cementing the sand grains together. Subsequent erosion of the uncemented bulk of the Tertiary beds left these hardened patches behind to form the sarsens or greywethers, which litter the landscape in these parts of the county. Many have been broken up and removed over the centuries as a convenient source of building materials, from the Stone Age onwards, but some patches remain and now have conservation protection."