Geology Site Account

Site name: Arena Essex Sand Pit

Grid reference: TQ 587797

Brief description of site:

Former sand pit of geological interest with an important collection of sarsen stones around the site. Formerly the Arena Essex Motor Sports Complex.

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Summary of geological interest

On the south side of the A13, in a large sand pit that was formerly the Arena Essex Motor Sports Complex, is a fine collection of at least 20 large sarsens with exceptionally well-preserved surface structures. The largest stone is over three metres long and is almost certainly the largest sarsen in south Essex.

Most of the stones are piled up in one place close to the car park at TQ 5875 7967 but others are scattered about. One stone is protruding from a bank of what appears to be Thanet Sand. This may be a unique in-situ example.

The origin and formation of sarsen stones

The origin of sarsens appears to be the sands of the Woolwich and Reading Beds, which occurs above the Thanet Sand. At some stage these sands must have been exposed at the surface when the climate of Essex was extremely hot (probably during the Palaeocene Period some 55 million years ago) and during that time water with dissolved silica was drawn to the surface. The sand therefore became cemented by silica (in the form of quartz) to form a tough layer of sandstone called silcrete. Subsequently, when these beds were subjected to erosion and broken up, large blocks of these hard rocks remained.

Compared to sarsens elsewhere in Essex the sarsens of Thurrock have not been abraded and retain their remarkable mammilated surfaces which are growth structures formed as the quartz slowly crystallised between the sand grains. The preservation of these surfaces is due to the fact that, although the stones were picked up and transported by the Thames during the Ice Age, they have not travelled far. They came to rest in the Thames gravels which here lies on top of the Thanet Sand.