Geology Site Account

Site name: Asheldham Pit

Grid reference: TL 973 017

Brief description of site:

A disused pit in the Asheldham Gravel, laid down by the Thames-Medway River during the Ice Age. The pit has exposures of gravel and is of educational and scientific interest.

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Summary of the geological interest:

The ribbon of flooded gravel pits stretching across the Dengie Peninsula from Burnham-on-Crouch to Bradwell-on-Sea was the route of the Thames-Medway river and the gravel it deposited is known as the Low Level East Essex Gravel. One of these pits, at Asheldham, is a former nature reserve where the iron-stained sandy gravel is visible in the banks.

This gravel, known locally as Asheldham Gravel, was laid down about 350,000 to 400,000 years ago during the Ice Age. For most of this time sea level was very low and the Thames joined the River Medway in what is now the Southend area and flowed north-east as a combined river towards Clacton and out across what is now the southern North Sea to become a tributary of the Rhine.

The gravel consists mostly of flint pebbles but it also contains a mixture of rocks from Kent (brought here via the River Medway) and from north-west England (carried down the main Thames valley). Study of the gravel constituents has helped geologists to unravel the complex histories of these two great rivers.

The gravel has also provided evidence for a later, exceptionally cold, period of the Ice Age. The upper layers have been disturbed by the effects of ice such as frost cracks and ice wedges - evidence of a permafrost landscape such as that seen in Arctic areas today.

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Scientific interest and site importance

Asheldham Pit (originally known as Cawoods Pit) was formerly a nature reserve leased to Essex Wildlife Trust. A lake occupies about one third of the site. Plant and insect life is varied and large numbers of wildfowl winter on the lake. A permanent vertical section through the gravel has been created at the western end of the reserve, which shows 'current-bedding' where underwater currents have deposited sand and gravel in inclined layers in the direction of flow.

Asheldham Pit is important because nowhere else on the Dengie peninsula can deposits of the Thames-Medway river be examined in safety. The gravel provides an opportunity to see the evidence of a river, which had a great influence on the landscape of Essex. The gravel consists of a number of different rock types, which provide clues to the routes of the Thames and the Medway rivers during the Ice Age.

Asheldham Pit is the type locality for the Asheldham Gravel, a coarse-grained ferruginous sand and gravel. The deposits are typical of the post-diversion Thames-Medway river, deposited by the Thames, downstream from its confluence with the Medway, as it flowed across this part of Essex. Palaeocurrent measurements from this pit indicate a flow to the north-east (Bridgland 1983).

The deposits are of some contention. Gibbard et al. (1996) regards them as late Anglian in age, the uppermost member of a sequence of deposits reflecting the formation of a lake in the southern North Sea, caused by damming by the Anglian ice sheet, and the sudden emptying of this lake by the breaching of the Straights of Dover. Bridgland (1994), however, regards the gravels as belonging to a more recent cold stage than the Anglian and that they are the upper part of a complex series of channel deposits.

Stone counts from the site in the 1970s revealed the Asheldham Gravel to consist of 13 to 17 per cent Lower Greensand pebbles, brought here from Kent, although some counts have an unusually high percentage of such pebbles which is difficult to explain.

In the 1970s the upper 3 metres of gravel was seen to consist of reddish brown sandy loam with evidence of frost action in the form of festooning, frost cracks and fossil ice-wedge casts, overlying about 3 metres of cross-bedded sandy gravel. The ice disturbance structures (cryoturbation) probably date from the Devensian, the most recent glacial stage. The bedrock beneath the floor of the pit is London Clay.

Humans were no doubt present here during the warmer periods, the evidence for this being flint hand axes from the upper gravels at Goldsands Road Pit, Southminster.

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Other information

Southminster railway station is within walking distance (about 3 km). Asheldham Pit was used as a BBC filming location for Doctor Who in 1972.