Sarsen stones are the post-glacial remains of a cap of Cenozoic silcrete that once covered much of southern England. This is a dense, hard rock, created from sand bound by a silica cement, making it a kind of silicified sandstone. This is thought to have formed during Neogene to Quaternary weathering by the silicification of Upper Paleocene Lambeth Group sediments, resulting from acid leaching. Sarsen stones can still be found across much of central southern England.
Between 60 and 40 million years ago, silt and sand were laid down, either as river-borne alluvium or as marine sediments, across parts of southern England. Then, around 35 million years ago, these layers of sand were covered up by additional deposits and were subsequently saturated with silica-rich groundwater.
Over time, the concentration of silica increased further – possibly as a result of increases in acidity or the evaporation of the water in which it was dissolved. When the water could no longer hold the silica in solution, the mineral precipitated out and filled the pores between the grains of sand. As the precipitation continued, the silica began to act as a mineralogical glue, sticking trillions of grains of sand together to form a two-metre-thick layer of solid rock, known as sarsen, immediately below what had originally been the water table.
Over the past three million years – particularly during the last ice age – water erosion and freeze-thaw processes exposed, eroded and broke up this crust of sarsen into millions of loose boulders that remained scattered across the landscape.
The word "sarsen" is a shortening of "Saracen stone" which arose in the Wiltshire dialect. Perhaps the most famous Sarsen stones are those at Stonehenge, used for the heelstone and the sarsen circle uprights. Many other prehistoric monuments in southern England feature Sarsen stones, for example Avebury. Sarsens are renowned for their durability, leading to use as, for example, kerbstones. (Some of this information has been taken from Wikipedia and some from the magazine of the Royal Geographical Society.)
The Sarsen stone here has been carved to commemorate the Diamond jubilee of Queen Elizabeth II in 2012.
The indicated parking is at the village hall and it is only a short walk to the cache site.
To qualify for this earthcache, please answer the following questions and send to us via our profile:
1) The text above describes the breaking up of a rocky crust. Describe the surface appearance of this Sarsen stone and say whether this is consistent with the text. If not, why might it differ?
2) The area of the stone that has been smoothed for the carving is paler in colour than the rest of it. Why might this be?
3) what are the approximate dimensions of the carved Sarsen stone (height, width, length)?
4) (optional) please post a picture of you or your GPS with the Sarsen stone
5) (optional) please describe where else you have spotted a Sarsen stone in or around Chilton - there are lots of examples