Abstract
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.
Highlights
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 ex tension of the glacier in the same way, but is characterized by marked discontinuities
The present paper demonstrates on the base of obser vations in the area of the Iller glacier of the last glaciation that Pleniglacial glaciers with positive mass budget have moved in another way than present-day gla ciers with their - mostly - negative mass budget
It is characterized by a summa tion 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 un derground is possible only where the glacier ad vances beyond its stagnant-ice basement, i. e. immediately behind the front of the advancing glacier
Summary
Wie glaziale Erosion vor sich geht, ist teils seit über einhundert Jahren bekannt, teils bis heute ungeklärt und daher umstritten. Vergegen wärtigt man sich schließlich, daß der Wechsel von Becken und Schwellen nicht nur die wasser-(und eis-)scheidennahen Bereiche des Hochgebirges be trifft, sondern sich ü b e r ein gestuftes Längsprofil (Abb. 3 und 4 ) in die inneralpinen Talgründe und unter deren Sedimentfüllung1) bis ins Alpenvorland fortsetzt (Abb. 5 ) , dann wird klar, daß glaziale Erosi on (und Übertiefung) jedenfalls kein über die ge samte Länge der pleistozänen Gletschererstreckung gleichmäßig wirksamer Vorgang, sondern durch ausgeprägte Diskontinuitäten charakterisiert war. Abb. 2: Das vom Gletscher seit dem Hochstand von 1850 geräumte übertiefte Becken des Sulzenauferners oberhalb der Sulzenau-Hütte. Gegebenheiten im eisscheidennahen Bereich heuti ger Gletschertäler, so erscheint es auf den ersten Blick durchaus plausibel, auch wenn man nicht die zumeist benutzten überhöhten, sondern nicht-über höhte Profile zum Vergleich heranzieht (Abb. 4 ) : die Gefällssprünge sind teilweise so kräftig, daß ein Wechsel von Dntckentlastung am Kopf, Drucküber lastung am Fuß der Gefällssteilen, wie ihn das Mo dell unterstellt, als Erklärung für die beobachteten Phänomene denkbar erscheint. Das obere Profil zeigt, data der Rheingletscher - wie alle Vorlandgletscher - im Verhältnis zu seiner Flächenausdehnung außerordentlich dünn war, sein Oberflächengefälle gering und auch das zu überwindende Gegengefälle des präquartären Untergrunds nur schwach geneigt
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