The BGS Lexicon of Named Rock Units — Result Details
Colchester Formation
| Computer Code: | CCHR | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Early Pleistocene (QPE) — Mid Pleistocene (QPM) | ||
| Lithological Description: | The formation encompasses fluvial, lacustrine and organic deposits of the pre-diversionary River Thames. Most of the surviving deposits are fluvial gravels, with sedimentary structures indicating deposition by a braided river. Lacustrine silts and clays and organic peats are uncommon. The gravels of the formation are characterised by quartz and quartzite from the Triassic, Carboniferous and Devonian rocks of the West Midlands, Welsh borderland and possibly southwestern Pennines, and felsic volcanic rocks from northern Wales. These lithologies form a lower percentage than in the Sudbury Formation, and are believed to be derived from that formation, but mega-erratics and felsic volcanic rocks from Wales and the Welsh borderland occur with a generally greater frequency than in the Sudbury Formation. The fluvial gravels occupy terrace levels and the members are defined on the basis of altitude and pebble clast content. The members comprise bodies of cross-bedded and massive, moderately sorted sand and gravel. The aggredations are generally entrenched into bedrock with a difference in surface elevation in the order of 5m. Intraformational ice wedge casts and pollen indicate periglacial, cool temperate and warm temperate climates. | ||
| Definition of Lower Boundary: | Unconformable on bedrock: Cretaceous, Palaeogene and Crag Group. Differentiation from the Crag Group can be difficult where the latter is reworked, and the distinction must be made on the basis of sedimentological structures: the Crag Group is marine, the Kesgrave Catchment Subgroup is fluvial. | ||
| Definition of Upper Boundary: | Commonly overlain by Middle Pleistocene glacigenic deposits. Upper boundary may be difficult to determine where overlain by glaciofluvial sand and gravel, but the presence of more angular clasts, chalk, and poorer sorting in the latter is usually helpful. The uppermost 1.0-1.5m of the formation is commonly affected by the Valley Farm Soil, a rubified and clay-enriched horizon, and/or by the Barham Arctic Structure Soil, a complex pedogenic horizon (Rose et al., 1985). | ||
| Thickness: | Individual terrace aggradations are typically 5-12m thick, total thickness may locally reach c.21.3m. | ||
| Geographical Limits: | The formation is restricted to the Thames Valley, Essex and Suffolk. It does not extend into Norfolk. (Hamblin and Moorlock, 1995). | ||
| Parent Unit: | Kesgrave Catchment Subgroup (KGCA) | ||
| Previous Name(s): | Essex White Ballast [Obsolete Name and Code: Use SBRY, KES, CCHR, KGCA]
(-3284)
Kesgrave Formation [Obsolete Name and Code: Use KES, CCHR, KGCA, SBRY] (-1621) Kesgrave Sands and Gravel [Obsolete Name and Code: Use CCHR] (-2063) |
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| Alternative Name(s): | Waldringfield Member Ardleigh Member Wivenhoe Member Lower St Osyth Member |
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| Stratotypes: | |||
| Type Area | The course of the pre-diversionary River Thames from Chelmsford to Colchester, Essex. Whiteman and Rose, 1992. | ||
| Reference(s): | |||
| Whiteman, C A and Rose, J. 1992. Thames river sediments of the British Early and Middle Pleistocene. Quarterly Science Reviews, Vol.11, 363-375. | |||
| West, R G. 1980. The Pre-Glacial Pleistocene of the Norfolk and Suffolk coasts. [Cambridge: Cambridge University Press.] | |||
| Rose, J, Allen, P, Kemp, R A, Whiteman, C A and Owen, N. 1985. The early Anglian Barham Soil in southern East Anglia. 197-229 in Boardman, J (editor), Soils and Quaternary landscape evolution. [Chichester: Wiley.] | |||
| Kemp, R A. 1985. The Valley Farm Soil in southern East Anglia. 179-196 in Boardman, J (editor), Soils and Quaternary landscape evolution. [Chichester: Wiley.] | |||
| Kemp, R A. 1987. Genesis and environmental significance of a buried Middle Pleistocene soil in eastern England. Geoderma, Vol.41, 49-77. | |||
| Hamblin, R J O, and Moorlock, B S P. 1995. The Kesgrave and Bytham Sands and Gravels of eastern Suffolk. Quaternary Newsletter, No.77, 17-31. | |||
| Whiteman, C A. 1992. The palaeogeography and correlation of pre-Anglian-Glaciation terraces of the River Thames in Essex and the London Basin. Proceedings of the Geologists' Association, Vol.103, 37-56. | |||
| Rose, J. 1994. Major river systems of central and southern Britain during the Early and Middle Pleistocene. Terra Nova, Vol.6, 435-443. | |||
| Rose, J, Whiteman, C A, Allan, P and Kemp, R A. 1999. The Kesgrave Sands and Gravels: 'pre-glacial' Quaternary deposits of the River Thames in East Anglia and the Thames Valley. Proceedings of the Geologists' Association, Vol.110, 93-116. | |||
| Green, C P, Hey, R W and McGregor, D F M. 1980. Volcanic pebbles in Pleistocene gravels of the Thames in Buckinghamshire and Hertfordshire. Geological Magazine, Vol.117, 59-64. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E207 E208 E222 E223 E224 E225 E240 E241 E242 | |||