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lignitic

Lambeth Group

The Lambeth Group, also known as the Reading Beds and the Woolwich Beds, is a geological formation comprising a complex of vertically and laterally varying gravels, sands, silts and clays deposited between 56-55 million years before present during the Lower Eocene Epoch. It is found throughout the London Basin with a thickness between 10m and 30m and the Hampshire Basin with a thickness between 50m and less than 25m. Surface outcrops are present only on the outskirts of these, however, that the Lambeth Group underlies some 25% of London at a depth of less than 30m means the formation is of great engineering interest for tunnelling and foundations.

The formation was first known as the Plastic Clay by T. Webster in 1816 after the Argile plastique of Georges Cuvier and A. Brongniart. It was called the Mottled Clay by J. Prestwich 1n 1846, but in 1853 he proposed the name Woolwich-and-Reading Beds to emphasise the differing local aspects of the series. This name received widespread usage, however, has in turn been recently deprecated in 1994 in favour of the Lambeth Group by the British Geological Survey in order to conform with new standards and to allow scope for more detailed subdivisions.

Three distinct types within the formation are recognized: (1) The Reading type, a series of lenticular mottled clays and sands, here and there with pebbly beds and masses of fine sand converted into quartzite. These beds are generally unfossiliferous. They are found in the north and west portions of the London Basin and in the Hampshire Basin. (2) The Woolwich type, grey clays and pale sands, often full of estuarine shells and in places with a well-marked oyster bed. At the base of the shell-bearing clays in southeast London there are pebble beds and lignitic layers. The Woolwich beds occur in west Kent, the east borders of Surrey, the borders of east Kent, in south Essex and at Newhaven in Sussex. (3) A third type consisting of light-colored false-bedded sands with marine fossils occurs in east Kent. Where it rests on the Thanet beds it is an argillaceous greensand with rounded flint pebbles; where it rests on the chalk it is more clayey and the flints are less rounded and are green-coated.

Except in the Hampshire basin the Lambeth Group usually rests on the Thanet beds, but they are found on the chalk near Bromley, Charlton, Hungerford, Hertford, Reading, etc. In Dorset the Reading beds appear on the coast at Studland Bay and at other points inland. The Hertfordshire puddingstone is a well-known rock from near the base of the formation; it is a flint pebbly conglomerate in a siliceous matrix. The fossils, estuarine, freshwater and marine, include Corbicula cuneiformis, C. tellinella, Ostrea bellovacina, Viva parus lentus, Planorbis hemistoma, Melania (Melanatria) inquinata, Neritina globulus, and the remains of turtles, crocodiles, sharks, birds (Gastornis) and the mammal Coryphodon. Bricks, tiles and coarse pottery and occasionally firebricks have been made from the clay beds in this formation.

References

  • Hight, DW, Ellison, RA & Page, DP, 2004, Engineering in the Lambeth Group, Construction Industry Research and Information Association, Report C583
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