Geology Site Account
Davy Down Riverside Park Sarsen Stone, SOUTH OCKENDON, Thurrock District, TQ59268004, Potential Local Geological Site
Outside the pumping station at Davy Down Riverside Park is a remarkable sarsen stone – probably the finest example in Essex. The sarsen is 1.6 metres square and has fine mammillated surfaces. It formerly stood outside Marley’s works on the south side of Stifford Road, South Ockendon and is understood to have been found in the gravel pit south of the works.
Davy Down Riverside Park forms part of the Mar Dyke Valley, a delightful valley with steep, wooded sides and a grazed floor that can be seen from the M25 motorway (see below).
Also of interest at Davy Down is the pumping station, built in 1928 with a 50 metre deep borehole down into the Chalk. It is still used today for pumping water for use in the local area.
Davy Down Riverside Park is open at all times but the pumping station and the sarsen stone are only accessible during opening hours when the warden is present.
The formation of the Mar Dyke Valley
The Mardyke valley runs along the northern side of the Purfleet Anticline, an east-west trending ridge of chalk between Purfleet and Grays. However, the Mar Dyke river that occupies this valley appears to be much too small to have cut this wide valley.
A clue to solving this puzzle was an ancient gravel called Corbets Tey Gravel, which lines the Mar Dyke valley but also occurs between Ilford and South Ockendon and is without doubt a Thames terrace deposit. Geologists have therefore now concluded that, this valley was, remarkably, originally formed by the Thames when it was flowing in a south-westerly direction. It appears that the Thames, some 300,000 years ago, was flowing eastwards from what is now London when it turned sharply south-westwards in the South Ockendon area because of the higher ground of the Purfleet Anticline. It then flowed through what is now the Mar Dyke valley to Purfleet and then swung southwards through a narrow gap in the chalk ridge to resume its eastward course. The modern Mar Dyke has therefore adopted an abandoned part of the old Thames valley. This picture has been built up by painstaking examination of gravels which not only tell the source of the river that deposited them but also the direction of the current at the time.
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Reference: Bridgland 1994 (p. 218-224)
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