Geology Site Account

Site name: Greenstead Sarsen Stone

Grid reference: TM 0231 2483

Brief description of site:

A sarsen stone stands by a bus stop in Avon Way. It is very unusual to see a sarsen stone in such an urban setting.

Details

A fine large fragment of a sarsen stone (0.9 x 0.9 metres in size) stands upright by a bus stop in Avon Way. It is the largest of several sarsen stones in the vicinity, presumably all found when the housing estate was constructed in the 1960s.

About 150 metres to the west of this stone is a boulder of Hertfordshire puddingstone - see separate site entry (Greenstead Puddingstone - TM 0216 2481).

The origin of sarsen stones

Sarsens are extremely hard boulders of sandstone formed around 55 million years ago when the climate of Britain was hot and a layer of sand beneath the surface of the ground became cemented with quartz. They are thus very resistant to erosion and have survived the rigours of the Ice Age. They originated on the chalk downland north and west of Essex and were carried here by rivers and glaciers. After retreat of the ice they became concentrated in river valleys.

The formation of silcretes (which includes sarsens and puddingstones) has been the subject of recent scientific debate. Research has compared the conditions under which sarsens and puddingstones may have been formed with the present day climate in the Kalahari Desert and parts of Australia.

The Greenstead Sarsen Stone by the bus stop. Photo: G. Lucy