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Original Article |
1 1Godwin Institute of Quaternary Research, Department of Geography, University of Cambridge, Downing Street, Cambridge CB2 3EN, UK (e-mail: plg1@cus.cam.ac.uk)
2 2Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 2DB, UK
The evolution of the drainage system of lowland Britain is discussed on the basis of available geological evidence, including that from terrestrial sites and that which has more recently become available from offshore exploration of the North Sea, the English and Bristol Channels, and the Irish Sea. Tertiary stratigraphy throws considerable light on landform and river development. Paleocene destruction of a chalk cover, which seems to have been incomplete in western Britain, was accompanied by basin sedimentation under a tropical climate. The major elements, the Thames, Solent, Hampshire (?proto-Avon) river, Irish Sea river and possibly an early Trent river, existed almost throughout the Cenozoic. The influence of Atlantic rifting and thermal doming in NW Britain appears to have been stronger and more temporally focused than the persistent flexuring that determined and maintained Tertiary drainage lines in the SE. Here also the folded Mesozoic terrains on the surface contrast with the more dominant block-faulted relief of the Palaeozoic ‘oldlands’. The rivers of the SE can be shown to have extended or reduced their lengths in response to relative sea-level change and gentle warping. Drainage antecedence, the destruction of the Solent system and the breaching of the English Channel are also evident. By contrast, the major river systems of the west are now entirely submerged. Long-term stability of the drainage pattern reflects a persistent tectonic regime in the south, with a subdued low-relief landscape having a weathered regolith and dense vegetation cover. Meandering river channels and alluvial styles predominated, although channel forms varied according to sediment load, slope and discharge variability. Coarse gravel-dominated accumulations are rare and localized. Chemically stable lithologies dominate the clastic component throughout. It is apparent that the deeply incised river valleys seen today are related to high, predominantly coarse sediment yields, encouraged by substantial, rapid climate changes in the Pleistocene. This emphasizes the significance of mechanical compared with chemical weathering for the rate and nature of landscape dissection, and the modifications that have arisen as a result of glaciation, frost-climate weathering, rapidly changing climates and sea levels. The stratigraphical evidence here reviewed is at variance with older, largely geomorphologically based landform evolution models (‘denudational chronology’), but gives considerable support to the recent proposals emphasizing the significance of Paleocene erosion, and enduring low-relief landscapes and drainage systems evolving alongside fold development during the Paleogene. Given the depobasin evidence now available, postulated fluvially active episodes can, and must, be linked to contemporaneous deposition. Some at least of the many controversies involving the identity of erosion surfaces, the dating of them using only residual deposits and weathering mantles, and the selection of particular Tertiary episodes as ones of landscape development can now be resolved.
Key Words: Tertiary • rivers • stream sediments • drainage • landscapes
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