Level of confluence
A level of confluence is a level of terrain created in connection with a confluence - i.e. the confluence of glaciers - through glacial erosion . There are two different definitions of a so-called stage:
- on the one hand, a deepening of the glacier bed at the point of confluence due to the increase in mass and the resulting increased erosion effect;
- on the other hand, the step in the valley slope of the main valley to the opening of the hanging side valley of the smaller glacier flowing towards it .
A confluence stage is the counterpart of a diffluence stage that can occur at points where a former glacier splits.
Deepening of the glacier bed at the point of confluence
According to this definition, which can be found almost exclusively in today's literature, the term denotes a steep terrain, especially in the main valley at the point where two or more glaciers meet. This steepening is due to a deepening of the glacier bed, since glacial erosion , especially deterion , has increased here due to increased ice pressure and greater flow velocity . This level is particularly pronounced when the confluent glaciers are roughly the same size. It should be noted, however, that if the two valleys do not become ice-free at the same time when two glaciers converge, the later retreating glacier blocks the other valley and this is then "clogged" with gravel and moraine material. It should also be noted that there are also other glacial processes that create depressions and thus also steps.
Valley slope to the hanging side valley
Albrecht Penck , who at the end of the 19th and beginning of the 20th century made a major contribution to the basics of the glacial valley shapes and the associated geomorphological processes, used the term confluence level, at least initially, exclusively for the ascent from the main valley to the confluence of the hanging side valley of the flowing smaller glacier. This becomes clear from the following quote:
The steps of confluence are seen in the hanging mouths of side valleys; the steps of diffluence are hanging openings of those valleys which were entered by a branch of the ice. The height of both kinds of steps will generally be more considerable, the greater the difference between the main glacier and its affluent or diverting branch.
“Stages of confluence can be recognized as hanging mouths of side valleys; Diffluence stages are hanging openings in the valleys into which a branch of the ice flow has penetrated. The height of both types of steps is fundamentally more important, the greater the difference between the main stream and the incoming or outgoing branch. "
In these stages, it is not the increase in erosion force that occurs in the immediate further course that is decisive, but the difference in erosion force due to the different glacier thicknesses.
Examples
The most striking stages of confluence mostly arise where slowly creeping snow ice fields meet and henceforth form a much faster moving ice stream. This is currently evident, for example, on the Pasterzenletscher in the Hohe Tauern (see picture). After the ice has melted , such stages of confluence are often characterized by high waterfalls, such as the beginning of the Königssee valley with the Röthfall .
If two or more glacier streams meet, this can also lead to increased erosion performance in the combined ice stream. After the glaciers melt, the rivers in the valleys in question form waterfalls or steep gorges before they meet. The inner-city Gastein waterfall of the Gasteiner Ache before the confluence of the Kötschachbach is well known. The neighboring Mallnitzbach overcomes in the Rabisch Gorge a common confluence level with the Dosenbach. In the southern Black Forest , too , the Menzenschwander and Bernauer Alb cut small, steep gorges into their confluence stage, which emerged in the last cold periods , before they meet. A terrain level where the interpretation as a confluence level is obvious can also be found in the Rhone Valley near Fiesch , where the Rhone Valley is clearly steeper at the point where the Fiescher Valley joins . During the last ice age maximum , the Rhone and Fiescher glaciers met here .
Hanging valleys with distinctive steps pile up along particularly large former glacier streams. Their high waterfalls are therefore particularly characteristic of fjord landscapes . Prominent examples are the Bridalveil Fall in Yosemite Valley , the side valleys of the Lauterbrunnen Valley in the Bernese Oberland or the Todtnau Waterfall in the Black Forest below the Todtnauberg high valley .
There can also be transitional forms between the two types. For example, in the upper Maggia valley near Cortino, about two kilometers above Lago del Sambuco , a small trough valley that originates north-west of Pizzo Sciresa joins in a step from the southwest . A little above this point, the main valley also has a step about 100 meters high.
Individual evidence
- ↑ a b c Eduard Gerber: The longitudinal profile of the Alpine valleys. In: Geographica Helvetica. Volume 11, 1956, pp. 160-215 ( online )
- ^ A b Albrecht Penck: Glacial features in the surface of the Alps. In: Journal of Geology. Volume 13, 1905, pp. 1-19 ( online ).
- ↑ Harald Zepp: Geomorphology: An Introduction. 5th edition, Verlag Ferdinand Schöningh, Paderborn 2011, ISBN 978-3-8385-3593-7 , p. 198 ( Google books )
- ↑ level of confluence at geodz.de
- ↑ level of confluence at Spektrum.de
- ↑ Julia A. Jackson, James P. Mehl, Klaus KE Neuendorf: Glossary of geology. Springer Verlag, Berlin 2005, ISBN 0-922152-76-4 , p. 179. ( Google books )
- ^ A b Hermann Lautensach: The deepening of the Ticino region. In: Geographical Treatises. Book 1, University of Berlin, 1912, p. 76
- ↑ Heribert Louis: On the theory of glacial erosion in valleys. In: Ice Age u. Present. Volume 2, 1952, pp. 12-24 ( online ( Memento from November 29, 2014 in the Internet Archive ))
- ↑ Frank Preusser, Jürgen M. Reitner, Christian Schlüchter: Distribution, geometry, age and origin of overdeepened valleys and basins in the Alps and their foreland. In: Swiss Journal of Geosciences. Volume 103, 2010, pp. 407-426 ( online )
- ^ Albrecht Penck, Eduard Brückner: The Alps in the Ice Age. Tauchnitz Verlag, Leipzig 1901–1909, Volume III, p. 811
- ↑ Map section with Rhone Valley near Fiesch at geo.admin.ch
- ↑ Map section with Maggia Valley at geo.admin.ch