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Sarsen stones are found scattered across the Downs between Avebury and Marlborough; some have also been found resting on Jurassic rocks near Swindon. There are few on Salisbury Plain, though there were probably many more, as they are found broken up as building stones. The name comes either from the Anglo Saxon sar stan, meaning “troublesome stone”, or from Saracen, meaning “alien” or “strange”. They were also known as “grey wethers” because of their resemblance to sheep. In prehistory, they were used in megalithic monuments such as Avebury and Stonehenge. Also, being extremely hard, they were used in the Neolithic as polissoirs, polishing stones for stone axes (and sometimes for the axes themselves). Grooves and rounded hollows can still be seen in individual sarsens at Totterdown and in the West Kennet Avenue.

Sarsen stones are white to greyish or yellowish sandstone, and are made of often iron-stained quartz sand, sometimes with flint grains and white feldspar. They often contain flint pebbles; when many are present, they are called “pudding stones”. They are probably remnants of locally hardened zones in sands of Tertiary age (maybe from the Reading Beds or Upper Bagshot Beds). They were possibly formed when groundwater became saturated with silica as it moved through sands, and when of local changes in chemistry caused cementation of the loose grains into silcrete: hard, silicified areas which developed beneath the ground surface, and just above the water table. Because cementation was not uniform, surfaces were knobbly and pitted; hollows formed by the later removal of softer areas. Some contain fossil roots, probably of plants such as palms; these roots may have become silicified, or replaced by sandy, iron-rich materials, or have disappeared leaving empty tubes. The Tertiary-aged cover of soft, uncemented sands and clays (which was probably continuous in Wiltshire), then eroded away, leaving just these locally silicified patches in linear “sarsen trains”, which formed along drainage lines as groundwater moved towards valleys. The removal of the softer enclosing sediments took place in Quaternary time, though it probably started in the late Tertiary. During wet, periglacial conditions, there was also probably downward movement of the sarsens into valleys, lubricated by slippery chalk mud.

Sarsens are extremely hard, but sandstone is softer when first exposed to the atmosphere. Three large masses of silica-cemented sandstone, similar in size to the Stonehenge sarsens, have been found in Oxfordshire in reddish Clay-with-Flints. Though silicified, they were still friable because they had not been exposed to the atmosphere for enough time. This suggests that the sarsens of Stonehenge would have been much more easily dressed if freshly uncovered. Sarsens also ‘sweat’ in damp weather by condensing atmospheric water vapour on to their surface.