Our area’s geology is very recent; there are no old rocks anywhere near the surface. Some 40 million years ago during the Eocene period the whole area lay beneath the sea – see simulated view from space. Over millions of years,
SE England under the sea 20 million years ago
nearby sandstone hills eroded and the debris was deposited by rivers on the sea floor. Some 200m of sand built up. In addition, glaciers scrapped gravels in large amounts from the land and dropped it in the sea. So the geology of Chobham is all about sand and gravel.
When Britain tilted to the NW, this area was raised again above sea level. The upper layers of sand were eroded and rivers cut down into the lower layers. Gravel layers tended to protect from erosion; hence the isolated hills and the highest ground tends to have these caps.
Nutrients are easily leached from sandy soils so they are very unfertile – hence large areas of the land around Chobham is poor heathland. It is only in the river valleys that alluvial soil deposited by the rivers provides soil that is fertile enough to sustain farming over a long period.
The diagram shows a simplified cross section of the area from the hills to the North to the low ground towards Woking.
Chobham village lies on a low east-west gravel ridge surrounded by alluvial soils which follow the courses of the Bournes. Going north, as the ground rises, the alluvium disappears and the Bagshot Beds are encountered at Burrowhill. The ground continues to rise further north and west where the Bracklesham Beds are encountered at Chobham Place. The higher Barton Beds have been eroded away over almost all the area; outcrops can be found on the high ground linking Staple Hill, Tank Hill and Oystershell Hill. Here and there, plateau gravel appears; this slow-to-erode soil is normally associated with isolated heights (such as Chobham Place, Jubilee Mount, Staple Hill and Tank Hill).
These are mainly fine white, buff sands with local beds of flint-pebble gravel. The full thickness probably does not exceed 40m. The Bagshot sands are visible in the area immediately north of Chobham village; around Burrowhill, Larkenshaw, Chobham Park, Gracious Pond, and all the way up to Longcross. Elsewhere, in the river valleys, they lie under alluvial material except at Focklesbrook which lies on an island of Bagshot sand.
The larger medieval farms appear to be centred on exposures of the Bagshot sand.
The hollowed out pond area at Burrowhill was once a sand quarry. Bagshot Sand locally contains a 1 metre seam of exceedingly fine sand, used for polishing. Inhabitants of Killy Hill report that if they dig in their gardens they come across fine almost-white sand. The Sand and Silica Mining Company at Burrowhill, although on a lower Bracklesham bed, mined this fine sand.
Fossils are confined to pieces of lignite and plant fragments and a few casts of marine gastropods.
These beds were laid down as alternate layers of sand and clay. The lowest layer is of clay, usually laminated and lilac tinted but sometimes brown; a middle division consisting of highly glauconitic sand, often deep green with seams of variegated plastic clay; and the highest of sand, loam and clay, with in places, a pebble-band at the base. The total thickness is from about 13 to 20 m.
The mix of clay and sand was once popular for brick making. Both the former brickworks at Brick Hill and Titlarks Hill (south of Sunningdale) were located in the Bracklesham Beds.
An ironstone band sometimes occurs at the junction with the Bagshot Beds and along the upper Windle Brook. This may have been the source of ore for the prehistoric iron industry in the upper Windle Brook.
The middle division occasionally contains fossils.
These are fine-grained, level-bedded, yellow sands, frequently marked off from the strata below by a pebble-bed and with occasional loamy seams near the top. The original thickness of this bed is at least 17m. The presence of churt in the pebble-bed is of interest, as it indicates that the denudation of the Weald had by then progressed so far as to uncover the Lower Greensand.
A ridge of Barton sands runs from Chobham Place to Staple Hill.
Fossils are found only very occasionally and are marine.
In some of these sand beds, percolating water cemented together areas of sand which became loose boulders when the sand around them became eroded. Sarsen and iron pudding stones have been used in the building of St Lawrence and Bisley churches – see photo. Sarsens can most easily be found “in the wild” on the heathlands whilst the pudding stone is found along the Bourne upstream of the Town Mill at Chobham.
Very few fossils are found in the sandy beds of our area. Oystershell Hill on the north Chobham Common is said to be so named due to the fossilised oyster shells found there. But no examples are known to exist.
Mop, of Suttons Bakery, found a fossilised sea urchin (an echinoid – Conulus) in the Poors Allotment he works beside Sandy Lane in Burrowhill. It is a flint inner cast and so must have been formed in the chalk during the Cretaceous period – approximately 100 million years ago. Flint pebbles and fossils generally would have travelled a fair distance from the chalk hills to the west before being deposited in the sands at the bottom of the sea where our heathlands formed – yet curiously the specimen shows no sign of wear; it is even possible to see the individual scales. Bronze-age people collected fossilised sea urchins from the chalk and often placed them in graves. Perhaps this is how it got to Chobham in such pristine condition?
The following sources were used in developing this page:
“London and the Thames Valley”, published by the National Environmental Research Council, Institute of Geological Sciences.
‘Britain Before Man’, The Geological Museum.
The Geology Museum in South Kensington and HMSO are good sources of geological books.