Rock avalanche dating in the Valais, Switzerland

Abstract

The Saaser Vispa valley in the Valais, Switzerland, is a deglaciated valley in the Alps that has been affected by ancient rock avalanche (Bergsturz) events. These rock avalanches have mainly occurred on west facing valley sides, due to rockslides along discontinuities within mica schists dipping at around 40º into the valley. The avalanches have modified the steep glaciated ‘U shape’ valley side into less steep slopes that have enabled forestry and pastures for cattle. The avalanche deposits have been colonised by vegetation including larch trees Larix decidua. The avalanches are probably due to paraglacial adjustment following deglaciation of the valley. The removal of glacial ice buttressing and resulting stress relief cause discontinuities to open, into which snowmelt and rainwater initiate weakening of the steep rock slopes until catastrophic failure occurs, resulting in rockslides and avalanches. It is probable that the rockslides have occurred within the last several thousand years. Lichenometry and dendrochronology are being used to estimate the age of the rock avalanche deposits.