The last ice age widened the Aare and Gu"rbe valleys

Date:
February 2, 2022
Source:
University of Bern
Summary:
A team was able to prove that the glaciers of the penultimate
ice age ('Riss' glaciation) mainly eroded the bedrock between
Thun and Bern, but that during the last glaciation (' Wu"rm'-
glaciation) glacial carving resulted in a widening and not in a
further deepening of the valleys. The researchers reconstructed
the geometry of the bedrock using gravity measurements to reach
their conclusions.



FULL STORY ==========================================================================
A team led by the University of Bern was able to proof that the glaciers
of the penultimate ice age ('Riss' glaciation) mainly eroded the bedrock between Thun and Bern, but that during the last glaciation (' Wu"rm'- glaciation) glacial carving resulted in a widening and not in a further deepening of the valleys.

The researchers reconstructed the geometry of the bedrock using gravity measurements to reach their conclusions.


==========================================================================
The glacial cycles repeat themselves every hundred thousand years. This
was also the case during the recent geological past in the Alpine
region. During the 'Riss' Ice Age, between 200,000 and 150,000 years
ago, the glaciers advanced far into the Swiss Plateau and shaped the
landscape. The same occurred during the 'Wu"rm' glaciation between
100,000 and 20,000 years ago, when several smaller ice advances and
then the large glaciers 20,000 years ago led to a further change of our landscape. These ice masses not only sculpted the hills and the mountains,
they also resulted in the formation of valleys and gorges several hundred meters deep, the so-called over-deepenings. These were then covered with
gravel and debris after the glaciers retreated, and therefore they are
hidden under the present surface.

The city of Bern is built on such an underground valley system. The same
is also the case for the Aare and Gu"rbetal valleys. There the bedrock
is hidden up to 200 meters below the valley floor. What do these valleys
look like now? Do they have a steep flank and a U-shaped cross-section,
or are they more V- shaped with gently sloping flanks? What is the shape
of the valleys that the two major glaciers formed? These questions have
now been answered in a study conducted by the Institute for Geology at
the University of Bern. The researchers were able to prove that glacial
erosion during the 'Riss' glacial period mainly led to the deep carving
of these overdeepenings. They were also able to show that the glaciers
of the last major glaciation 20,000 years ago did not further deepen
these valleys, but widened them. The study was published in the journal Scientific Reports.

Mapping of the overdeepenings using gravity data The overdeepenings are laterally bordered by the sandstones of the Swiss Plateau; it is the
Molasse bedrock. The overdeepenings themselves were filled with moraines, gravel and lake sediments after their formation. These deposits are unconsolidated and therefore they have a lower density than the Molasse bedrock. In fact, the backfills of these overdeepenings are about 20%
lighter than the Molasse bedrock. These small differences can, however,
be measured with a Swisstopo gravimeter. A gravimeter measures the
acceleration due to gravity at a survey point, and this depends on the
density of the subsurface.

"These devices are very carefully adjusted so that even the slightest deviations in the density distribution can be measured," explains
Professor Fritz Schlunegger, who initiated the study together with his colleague from the ETH Zurich, Professor Edi Kissling. The researchers
have now conducted gravity measurements across the Gu"rbe and Aare
valleys. They could document the occurrence of overdeepenings beneath
both valleys that are filled with debris and lake deposits. In the
Gu"rbe valley this structure is about 155 meters deep, whereas in the
Aare valley the overdeepening is at least 100 meters deeper. Thanks
to exploratory drillings, it was already known that such deepenings
are hidden underneath both valleys. With the gravity measurements,
however, we were able to show for the first time that the flanks of
these overdeepenings are nearly vertical and that their bottoms are
flat," explains Dimitri Bandou, who carried out the study as part of his dissertation. This allowed the team to document that the overdeepenings
have a U-shaped cross section. "Therefore, these depressions were formed
by glaciers," confirms Bandou.

Asymmetric cross-section The gravity measurements also showed that
the overdeepenings are made up of two segments. "In the upper part,
the flanks of the overdeepenings are significantly flatter than at
depth. Thus the upper part is wide and flat, and the lower part is
narrow and deep." The researchers explain this special geometry to the
erosion of the glaciers during an older and a younger ice advance. "There
have been several ice advances in the Alps during the geological past, including in the Bern area," explains Bandou. A very large glaciation
occurred during the 'Riss' Ice Age between around 200,000 and 150,000
years ago. "The glaciers of this Ice Age were larger and thicker than
the glaciers during the last Ice Age between 100,000 and 20,000 years," explains Schlunegger. Because of the greater thickness, the 'Riss'
glaciers were able to deeply carve into the bedrock. The younger 'Wu"rm' glaciers were less powerful, and glacial erosion mainly resulted in a
widening but not in a further deepening of these overdeepenings.

Erosional mechanisms not fully understood It is still not fully understood
why glaciers form overdeepenings. "In contrast to the formation of
valleys by rivers, we still cannot simulate the erosive effects of
the glaciers and, in particular, the formation of the overdeepenings
with modelling software packages," explains Schlunegger. Apparently,
there is a lack of detailed information about the bedrock surface and especially about the flanks of such overdeepenings. "Glaciers erode not
only in depth but also in width, and that is why valley flanks provide diagnostic information so that the erosional mechanisms of glaciers can
be better understood." The study that has just been published is expected
to yield in a better understanding of how glaciers shape the landscape.

This publication is the result of several years of collaboration
with the ETH Zurich, Swisstopo, the Stiftung Landschaft und Kies and
the Geba"udeversicherung Bern under the direction of Prof. Fritz
Schlunegger from the Institute of Geology at the University of
Bern and Prof. Edi Kissling from the ETH Zurich. The research
project was supported by the Swiss National Science Foundation (SNSF) ========================================================================== Story Source: Materials provided by University_of_Bern. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. D. Bandou, F. Schlunegger, E. Kissling, U. Marti, M. Schwenk, P.

Schla"fli, G. Douillet, D. Mair. Three-dimensional gravity modelling
of a Quaternary overdeepening fill in the Bern area of Switzerland
discloses two stages of glacial carving. Scientific Reports, 2022;
12 (1) DOI: 10.1038/s41598-022-04830-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220202111753.htm

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