history of the major rivers of southern Britain during the Tertiary, The

Journal of the Geological Society, Nov 2003 by Gibbard, P L, Lewin, J

Abstract: The evolution of (he 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. Paleoccne 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, Soient, 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 Soient 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 stratigraphicul 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 Paleoccne 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.

Keywords: Tertiary, rivers, stream sediments, drainage, landscapes.

Long-term drainage and landscape evolution have been important research foci for over a century. In southern Britain, investigations have centred on drainage evolution and its contribution to a 'denudational chronology' gcomorphological school, which reached its acme with the publication of Wooldridge & Linton's work Structure, Surface and Drainage in South-east England (Wooldridge & Linton 1939, 1955). Despite some criticisms (Pinchemel 1954) the conclusions reached therein reigned largely supreme until the 1970s when a new generation of researchers began questioning the conclusions on the basis of new evidence becoming available from increasingly sophisticated investigation of tectonic history (e.g. Ziegler 1987, 1994), sedimentology (e.g. Flint 1983a) and pedology (e.g. Catt 1983; Green 1985). The results continue to provoke reassessments of the geomorphology of southern England (Jones 1980, 1981, 1999?,/)) and beyond (e.g. Walsh etal. 1987, 1996; Walsh 1999; Mignon & Goudie 2001).

The approaches of Wooldridge & Linton (1939, 1955) and their immediate followers were essentially underpinned by three concepts.

(1) Gcomorphology could use deduction and inference to erect a history of landform development using evidence from landforms themselves to fill an apparent gap in Earth history after the rock record ceased. Only later on did the focus shift to a detailed reconstruction of processes and the stratigraphy of 'superficial' deposits.

(2) The Davisian cycle of erosion was the persuasive theoretical basis available for interpretation. In this, uplift was followed by progressive erosion and stream adjustment to geological 'structure' (including lithological outcrop) as relief reduced to an eventual peneplain (summarized by Davis 1909).

(3) Sea levels changed; this could include progressively falling base levels, and might involve marine onlap and offlap at particular periods. Ideas concerning superimposition from a chalk cover and the development of marine shorelines came to Britain from the work of Johnson (1919, 1931).