London Thames Basin

Abstract

The Thames system is the largest drainage basin in Britain: the Middle and Lower Thames occupying the London Basin, a syncline of Mesozoic and overlying Tertiary rocks. The Thames is a broadly W–E aligned stream axial to the basin, with tributaries entering from both the northern and southern margins. The catchment includes the south English Midlands, and in the east, the Estuary occurs where the river enters the North Sea. The deposits of the Thames and its tributaries occur from the tops of the highest hills on the basin margin (180 m OD) to below sea level. The highest Thames deposits, the Pebble Gravel Formation, represent the earliest course of the river, post-dating late Pliocene–earliest Pleistocene marine sands. A profound change in gravel lithology occurs in the next youngest units, which form a series of terrace remnants that are characterised by their content of rocks exotic to the present Thames catchment. These Kesgrave Formation units can be traced from the Upper Thames downstream through the Middle Thames Valley where they diverge from the modern course to pass into East Anglia where they form a terrace-like system, mostly buried beneath Anglian glaciation tills. This glaciation also overrode the Thames and its southbank tributaries north of London, resulting in the river adopting a new course through London. Subsequent evolution of the Thames system is marked by cyclic development of Maidenhead Formation gravel and sand aggradations under periglacial climates during the Middle and Late Pleistocene. The Thames terrace deposits also include interglacial fossiliferous sequences that provide both stratigraphical control and palaeoenvironmental evidence. East of London, the valley was invaded repeatedly by the sea so that, as today, a substantial estuary is developed during periods of high eustatic sea level (interglacials). In the estuary and upstream, a thick wedge of Holocene-age floodplain sediments have accumulated. Apart from the river deposits, slope and colluvial materials also occur, as well as periglacial materials, and the products of Chalk bedrock dissolution. Anthropogenic modification of the basin and especially the river’s course has been profound.