SUBGLACIAL VALLEYS IN THE LOWLANDS OF NORTH-WESTERN EUROPE

Fine examples of sub-glacial erosion and consequent sedimentation have recently been published from a series of North-American locations (Quaternary International 90-1, 2002). The magnitude of erosion and the transport capacity of the floodwaters have been gigantic as illustrated by the photographs of huge blocks of rock, dumped at the mouth of the valleys.

Deep incisions in unconsolidated sediments, interpreted as sub-glacial valleys, have been known for long in North-western Europe, in the southern North Sea, as well as onshore in The Netherlands, Germany, Denmark and Poland. These incisions can reach depths over 400 m below the former surface. Data have been presented by Wingfield (1990), who tied their formation to gigantic catastrophic floods. Later however, model studies by Van Dijke & Veldkamp (1996) showed that in the absence of height differences in the terrain large water volumes, flowing from the land ice cover, could erode sub-glacial valleys up till a depth of 50 m maximally. Praeg (1996) studied the floor of the North Sea from seismic surveys. He concluded that the deep valleys had been filled in an upstream direction. Erosion started near the margin of the land ice and continued in northern direction where the valley became deeper. Our study concentrated on the facies of the sedimentary infill of such a valley. The two drill-sites were located in the very northern part of The Netherlands at the southern continuation of a valley, observed in seismic lines in the North Sea, a few kilometres to the north. Clearly sedimentary sequences were present with changes from sands, ranging from 800 to 150 µm, coinciding with changes in gravel and clay content. In total 5 to 6 major sedimentation cycles were identified.

Comparison with similar features in Northern America points to a major difference in formation of the sub-glacial valleys. In North America the natural relief in the landscape has clearly been one of the causes in the process. In the lowlands of North-western Europe the only height differences were found within the land ice sheet.

The poster wants to discuss how huge quantities of water could be stored in the ice, how sub-glacial erosion could reach depths of 400 m below land surface and whether subglacial deformation is involved or not.