North West European Rivers
2. LATE PLIOCENE / EARLY PLEISTOCENE
Map 2. Late Pliocene - early Pleistocene (about 2,4 my) river courses.
The foundations of the modern drainage system were laid in the Miocene, when earth movements associated with both the Alpine Orogeny and the opening of the North Atlantic were at their height. Although greatly modified subsequently, the precursors of the modern rivers can be identified; indeed some appear to have been present since the beginning of the Tertiary. The form of this system was apparently closely-linked to geological structure and took a broadly simple pattern, although this may be an artefact of the preservation of the sedimentary sequences.
Throughout the Miocene and Pliocene, the rivers were generally transporting chemically resistant minerals and lithologies, such as quartz or flint, derived from long-term weathering under moist warm-temperate climates. Relatively little mechanical weathering is recorded except when associated with tectonic uplift, which resulted in rejuvenation of rivers such as the Rhine and gave rise to deposition of gravels. Otherwise, rivers seem to have occupied shallow channels and meandered across large regions with little evidence of deep valley incision. There is no indication of the development of permafrost, although some seasonal frost activity did possibly occur in the late Pliocene.
The onset of true cold climates in the Praetiglian (Pleistocene) Stage caused a marked change in depositional style with the input of gravel and sand derived by stripping of the deep regolith inherited from the Pliocene, and the adoption of a braided form by the rivers Rhine and Meuse. This however did not have great impact across the entire region, to judge from the apparently limited change in the Seine system. The return to temperate conditions in the Tiglian caused a readoption of the meandering river form and a renewed deposition of fine sediments.
In the Praetiglian, glaciation may already have been established in the Alps; ice-rafted blocks occur in Rhine sediments. By this time the Alpine Rhine may have been captured by the Upper Rhine system, having previously been flowing to the southwest to join the Rhone. Whether or not glaciation occurred elsewhere in northwest Europe at this time is not clear.