`Rivers cut deep notches in Alps` glacial valleys`
London: A new study sheds light on the processes that formed steep inner gorges in the broad glacial valleys of the Swiss Alps.
The U-shaped valleys were created by slow-moving glaciers. When they receded, rivers carved V-shaped notches, or inner gorges, into the floors of the glacial valleys.
New research led by a University of Washington scientist indicates that the notches endure, at least in part, from one glacial episode to the next. The glaciers appear to fill the gorges with ice and rock, protecting them from being scoured away as the glaciers move.
When the glaciers receded, the resulting rivers returned to the gorges and easily cleared out the debris deposited there, said David Montgomery, a UW professor of Earth and space sciences.
"The alpine inner gorges appear to lay low and endure glacial attack. They are topographic survivors," Montgomery said.
The river valleys can hide under the glaciers and when the glaciers melt the rivers can go back to work,” he added.
The researchers determined that, if the gorges were erased with each glacial episode, the rivers would have had to erode the bedrock from one-third to three-quarters of an inch per year since the last glacial period to get gorges as deep as they are today.
Such erosion rates would exceed those in all areas of the world except the most tectonically active regions, the researchers said, and they would have to maintain those rates for 1,000 years.
The team found other telltale evidence, sediment from much higher elevations and older than the last glacial deposits, at the bottom of the river gorges. That material likely was pushed into the gorges as glaciers moved down the valleys, indicating the gorges formed before the last glaciers.
"That means the glaciers aren`t cutting down the bedrock as fast as the rivers do. If the glaciers were keeping up, each time they`d be able to erase the notch left by the river," Montgomery said.
The work shows that “inner gorges can persist, and so the question is, ``How typical is that?`` I don`t think every inner gorge in the world survives multiple glaciations like that, but the Swiss Alps are a classic case. That`s where mountain glaciation was first discovered,” he said.
The research is published online December 5 in Nature Geoscience.
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