North West European Rivers
4. LATE 'CROMERIAN COMPLEX" STAGE (EARLY MIDDLE PLEISTOCENE)
Map 4: River systems during the latter part of the 'Cromerian Complex' Stage (during a glacioeustatic low sea-level stand).
The return to cold climates in the Eburonian-Menapian period established considerable frost weathering and therefore the loading of rivers with freshly weathered detrital material. The resulting periodic incision and later deposition under braided river regimes is well marked. The great increase in sediment deposition is recorded by the vast expansion of deltas in The Netherlands (Zagwijn I974). Glaciation seems to have become a widespread phenomenon, not only in the Alps, but also in upland Wales and in Scandinavia at this time. The result of the latter was the destruction of the Baltic River in the Menapian, after the possible formation of a proto-Baltic Basin by glacial scour.
The first records of the prevalence of true permafrost in lowland areas comes from the Dutch-Belgian-German border where cryoturbation of Praetiglian age has been observed.
By the later Middle Pleistocene the pattern of repeated incision and sediment
accumulation was well established. Indeed the change from the 40 ka to the
100 ka cyclicity appears to be accompanied by an important increase in the
intensity of supply of sedimentary detritus and thus a stronger incision
and sedimentation in river systems.
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. 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.