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On the genesis of the Aare gorge. Berner Oberland, Switzerland
Abstract
The gorge was not caused by epigenesis. Analysis of the direction of joints and river segments shows that it was predetermined by joints. The joints were cleaned out by erosion. There was very little downcutting. The joints reached down to the depth of the gorge. The pattern of the clefts was investigated, and their direction compared with the direction of river flow. From Innertkirchen to Meiringen widening was the main action. -D.J. Davis
... The Via Mala in the Grisons Alps and the Aareschlucht in the Bernese Alps (Fig. 7) are two famous examples of deeply incised gorges (Buxtorf, 1919; Hantke and Scheidegger, 1993). Interestingly, the geomorphological architecture is identical to the situation at Lammschlucht. ...
This paper explores the mechanisms by which inner gorges in the Alps were formed. It focuses on the ca. 1.5-km-long, 80-m-deep, and a few hundred meter wide Lammschlucht located in the northern foothills of the central Alps. We reconstructed the glacial cover using lateral moraines and hanging talus cones that record the elevation of the ice surface at the deglaciation stage of the LGM (Last Glacial Maximum). We used the reconstructed ice thickness patterns to calculate the erosional potential of the subglacial meltwater. The applied model is based on the principle of energy conservation and yields the pattern of downstream changes of the dynamic pressure, which is considered a measure for erosion potential. The model results suggest a maximum of the dynamic pressure at the end of the inner gorge. We interpret, therefore, that the subglacial meltwater scoured the reach toward the end of the Lammschlucht because of the enhanced dynamic pressure, which was ultimately controlled by the ice overburden. Post-glacial fluvial erosion then resulted in a readjustment through a regressive shift of the erosional front along the inner gorge farther upstream. The current location of this front lies almost in the middle of the Lammschlucht inner gorge where a step-pool channel changes into a straight plane-bed channel flowing on a deeply scoured bedrock. (C) 2011 Elsevier B.V. All rights reserved.
Overdeepened valleys and basins are commonly found below the present landscape surface in areas that were affected by Quaternary
glaciations. Overdeepened troughs and their sedimentary fillings are important in applied geology, for example, for geotechnics
of deep foundations and tunnelling, groundwater resource management, and radioactive waste disposal. This publication is an
overview of the areal distribution and the geometry of overdeepened troughs in the Alps and their foreland, and summarises
the present knowledge of the age and potential processes that may have caused deep erosion. It is shown that overdeepened
features within the Alps concur mainly with tectonic structures and/or weak lithologies as well as with Pleistocene ice confluence
and partly also diffluence situations. In the foreland, overdeepening is found as elongated buried valleys, mainly oriented
in the direction of former ice flow, and glacially scoured basins in the ablation area of glaciers. Some buried deeply incised
valleys were generated by fluvial down-cutting during the Messinian crisis but this mechanism of formation applies only for
the southern side of the Alps. Lithostratigraphic records and dating evidence reveal that overdeepened valleys were repeatedly
occupied and excavated by glaciers during past glaciations. However, the age of the original formation of (non-Messinian)
overdeepened structures remains unknown. The mechanisms causing overdeepening also remain unidentified and it can only be
speculated that pressurised meltwater played an important role in this context.
KeywordsGlaciations–Erosion–Sediments–Overdeepening–Alps–Quaternary
An intensive program of measurements of the orientation of joints in the field in Switzerland was undertaken in the Cantons of Graubünden, St. Gallen, Schwyz, Uri, Zürich, Zug, Aargau and Berne. For every «region», comprising 2-8 outcrops, the average orientation of the joint-parallelepiped was calculated by means of an electronic Computer based on a function-minimization program. If it is assumed that the steeply-dipping Joint surfaces have been produced by the present-day triaxial stress field according to the Mohr-Anderson theory of faulting, the orientation of the latter can be calculated. Thus, the net of present-day stress trajectories for Switzerland can be drawn. It turns out that the average orientation of the greatest compression in the region investigated is N 60°W. This agrees well with in-situ stress determinations by Gysel in the Sonnenbergtunnel near Lucerne (N 76.5°W) and by Greiner in southern Germany (N 28°-50°W). Converseiy, the hypothesis that most joints visible on fresh outcrops are due to the present-day tectonic Stresses and that the latter is homogeneous over extended areas, is thereby confirmed.