Climate - controlled glacial erosion in the unconsolidated sediments of northwestern Europe, based on a genetic model for tunnel valley formation

The development of large erosive subglacial forms in unconsolidated sediments is generally attributed to the eroding power of subglacial meltwater flowing under high pressure conditions. Most explanations, however, differ in the source of meltwater and the speed at which it erodes the subglacial bed. Based on the geometry of deep tunnel valleys and glacial basins in northwestern Europe, a reconstruction of subglacial hydrological conditions during the development of subglacial depressions is made. It is demonstrated that the flow of subglacial meltwater in subglacial channels under high glaciostatic pressures is only capable of eroding large volumes of sediment as long as there is imminent glaciohydrological instability. For the thick aquifers in northwestern Europe, this instability is achieved when large quantities of supraglacial meltwater are available. Furthermore, a theoretical definition is given for maximum depression depth to be reached by subglacial erosion. It is shown that this maximum depth is strongly related to average air temperatures during deglaciation and that glacier bed lowering is to be expected during any final phase of glaciations. The theoretical framework presented enables a tentative comparison between large-scale glacial morphology of different glaciations in northwestern Europe.