Before I write more about sarsen objects, sarsen tools and sarsen working, I thought I would summarise sarsen geology.
Sarsen is a silcrete sandstone, formed through the surface or near-surface silicification of other deposits. In Wiltshire, sarsen stones probably represent locally-silicified zones in the 4m to 5m thick Paleocene deposits overlying Cretaceous chalk; perhaps the Reading Beds or upper Bagshot Beds. The sand, silt and mud of these layers are thought to have been cemented by water-borne silica, aided by silica-rich plants growing at the time. It is possible that this silicification happened rapidly, over some 30,000 years or more. The cementation was, however, uneven. As the Paleocene deposits eroded, the scattered silicified patches were left on the surface. You can see these most clearly on the Marlborough Downs, in the Fyfield National Nature Reserve, where sarsen stones lie sleeping across the beautiful landscape.
Archaeologists tend to talk about two types of sarsen in Wiltshire. The most common – and the one of which the standing stones at Avebury and Stonehenge are comprised – is “saccaroid” sarsen. The less common is “quartzite” sarsen, which appears in the archaeological record as the principal hammerstone material at Stonehenge.
Saccaroid sarsen’s fresh break is white to grey in colour. The name is derived from the similarity of the fresh break’s surface to broken sugar loaf. It is made up of quartz sand grains and is usually found in large boulders.
Quartzite sarsen is usually a darker grey-brown colour, comprising much finer grained, clayey silts. It is usually found as nodules up to c. 60cm diameter. Fossils are rarely found in sarsens; but root voids and silicified roots are common, part of the characteristically gnarled look of much sarsen.
These two distinctions are useful although they mask sarsen’s subtle variations. For example, pebbles can be found cemented into the mix; varying amounts of iron oxide stain the stone red-brown; the cementation varies, leaving some grains of sand more-or-less well “glued” together. The “case-hardening” effect brought about by atmospheric weathering also affects texture and hardness. These variations can be found between stones and within individual boulders. And the shapes and sizes of sarsen stones vary immensely; from smooth, rounded boulders to irregular, contorted, pitted blocks.
Geddes, I. (2000, 2003) Hidden Depths: Wiltshire’s Geology and Landscapes Bradford on Avon: Ex Libris Press
Summerfield, M.A. and Goudie, A.S. (1980) “The sarsens of southern England: their palaeoenvironmental interpretation with reference to other silcretes” In Jones, D.K.C. (ed) The Shaping of Southern England Institute of British Geographers Special Publication 11 London: Academic Press