Über glaziale Erosion und Übertiefung
Abstract. Glacial erosion and overdeepening is - as can be shown by the longitudinal profile of every valley formed by glaciers - not a process active on the whole longitudinal extension of the glacier in the same way, but is characterized by marked discontinuities. The reasons have been discussed for nearly a century, but the problem remained unsolved until now. It can - as H. Louis (1952) has demonstrated - be solved only when the motion mechanism of the glaciers is known. The present paper demonstrates on the base of observations in the area of the Iller glacier of the last glaciation that Pleniglacial glaciers with positive mass budget (advancing glaciers) have moved in another way than present-day glaciers with their - mostly - negative mass budget. Furthermore it is demonstrated that this particular motion mechanism can actually be observed at advancing outlet glaciers of the Vatnajökull in Iceland. It is characterized by a summation of numerous push movements of flat ice-shields on shear-planes over stagnant-ice of preceding advances, all of them of relatively short duration and range. They originated in precipitation-caused mass surpluses in the feeding area. This motion mechanism implies that an impact on the underground is possible only where (and when) the glacier advances beyond its stagnant-ice basement, i. e. immediately behind the front of the advancing glacier. A larger amount of glacial erosion can be expected only when the glacier after the advance remained in the maximum position reached for a longer time and, additionally, (snow)meltwater under hydrostatic pressure could affect the underground. Further observations on meltwater movement within the glacier are presented from Iceland, on meltwater impact on the underground (mainly) from the German Alpine Foreland. To sum up, the discontinuities of glacial erosion and overdeepening can be explained by the motion mechanism of advancing glaciers described, the protective effect of the overridden stagnant-ice basement of preceding advances, and the characteristic movement of (snow)meltwater in, below and along the glacier. Because the erosion processes described take place (mainly) immediately behind the front of the advancing glacier, they are a phenomenon restricted in space as well as in time, but therefore must be of extraordinary intensity.