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North West European Rivers
10. CONCLUDING POINTS
The comparison of modern, deeply-incised river valleys with the apparently shallow features predominant during the earlier Neogene strongly suggests that alternating deposition and incision, which gives rise to these deep valleys, is a direct consequence of rapid climatic change. In particular it implies that frequent downcutting and aggradation in the region, where tectonic activity can be discounted, is a result of the occurrence of cold climates and the supply of abundant fresh materials by periglacial processes.
These observations are borne out by the sequences preserved throughout the region, in which interglacial sediments tend to be fine inorganic or organic materials intercalated in gravel and sand sequences. For this reason they have a low preservation potential and in general represent less than 10 % of the total fluvial sequences. The remaining 90 % of sediments are of cold-climate origin.
The effects of additional climatically controlled parameters are, with one exception, poorly understood. For example, the variation in amount of precipitation and its distribution through the year will certainly influence river flow.
The one exception to this general lack of knowledge is sea-level change. Glacioeustatically and isostatically controlled sea level changes are characteristic of the Pleistocene. In northwest Europe low sea levels during the cold periods caused great expansion of the drainage system onto the surrounding continental shelves. Indeed, deposition on the huge river floodplains of the North Sea and Channel areas almost certainly provided the major source of silt that formed the widespread loess deposits present in adjacent land areas.
For much of the Lower and Middle Pleistocene the southern North Sea was occupied by the huge delta complex of the North German Rivers, the Rhine, Thames, Meuse and Scheldt. This delta id often refrred to as the Ur-Frisia, or more recently as Eridanos delta. After the early Pleistocene, marine incursions during interglacial stages remained absent until the late Middle Pleistocene Cromerian Stage. A similar feature is found in the Channel Region. Marine transgression has subsequently been particularly marked during interglacial stages, and sea levels over 100 m below present are known from glacial maxima. Truncation of the greatly expanded fluvial system by marine transgression is very significant; the Channel River, for example, was approximately 800 km in length, 3.3 times longer than the present River Thames. Moreover, the drowned fluvial deposits are subjected to tidal scour, deposition of marine sediments and remobilization.
A further effect of marine transgression has been the infilling of the pre-existing river valleys by wedge-like accumulations of estuarine sediments. These sediments comprise silts, clays, and in some cases sands interbedded with peats and detrital organic sediments (e.g. Flandrian or Holocene). Interglacial examples are also widespread; in areas of subsidence they are buried by later deposits, whereas in relatively stable areas they are dissected by subsequent fluvial incision and remain as eroded remnants on valley sides.
It therefore is apparent that the impact of the various forms of climatic change on the northwest European drainage system has been very great. These changes have been superimposed on long-term climatic and geological trends resulting from plate tectonics and world geographical evolution over the past three million years or so. These trends might reasonably be expected to continue into the future.