Wide, U-shaped valleys in the Alps were gouged out by slow-moving glaciers over hundreds or thousands of years. During warmer periods, the glaciers receded, and rivers carved V-shaped notches, or inner gorges, into the valley floors. Geologists have long disagreed over whether subsequent glaciers erased these gorges or whether they persisted.

David Montgomery of the University of Washington and a colleague used topographic data taken from laser-based measurements to determine that, if the gorges were indeed erased during each glacial episode, the rivers would then have had to erode the bedrock at a rate of 0.8–2 centimetres per year since the last glacial period in order to create gorges as deep as those present today. ‘That is screamingly fast. It’s really too fast for the processes,’ Montgomery said.

The pair also identified sediment from much higher elevations, and older than the last glacial deposits, at the bottom of the river gorges. They argue that this material was probably pushed into the gorges as glaciers moved down the valleys, indicating that the gorges formed before the last glaciers.

It’s unclear how applicable the results are to other mountainous regions. ‘It shows that inner gorges can persist, and so the question is, “How typical is that?”,’ Montgomery said. ‘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.’

March 2011