On the origin of the drumlins of the South German Alpine Foreland (II): The sediments underneath

Abstract

The drumlins of the South German Alpine Foreland have certain characteristics: a distinctly limited occurrence within the formerly glaciated areas, a well defined time of formation and a typical, rather compact shape. The character of the sediments of which they are composed, and the relation between drumlin sediments and drumlin form have been a matter of discussion for more than 80 years due to the rareness of good exposures in drumlins. A few individual drumlins have become better known during the past decades, because they overlie interglacial deposits. The observations made at these sites allow the following statements: 1. (1) The drumlins normally consist of sediments of the last (Würm) glaciation. Only in exceptional cases do older Pleistocene strata or the pre-Pleistocene substratum (Molasse sandstone) form part of the drumlin body. These exceptions are important in so far as they prove that drumlin formation in principle was an erosional process in the area. 2. (2) The sediments are mainly till, but also a gravel/till sequence is not uncommon. The rare interglacial beds are mainly lake deposits. 3. (3) The thickness of the sediments on top of the interglacial deposits varies considerably: from some metres to several decametres. Even underneath a thin cover of younger sediments the fine-grained (and therefore in principle easily deformable) interglacial deposits are glaciotectonically undisturbed. Altogether these observations allow the conclusion, that (1) the process of drumlin formation shaped the glacier bed by highly selective, but locally vigorous, glacial erosion, and (2) this process did obviously not (or only poorly) affect the sediments below the ice/sediment interface even though it was composed of readily deformable material. Therefore the question is not, whether there was erosion or not but why there was not much more erosion and disturbance. It is suggested that the answer is likely to be found in the unevenness of the decaying permafrost table at the end of the Würm Pleniglacial.