The evolution of the moraine complex in the Fiescher Valley (Berner Alps, Switzerland) in the light of Schmidt-hammer exposure-age dating and sedimentological analysis

The sediment-landform assemblages preserved in many alpine valleys record glacier fluctuations during the latest Pleistocene and Holocene, encompassing the moraines formed during the Egesen stadial as well as Early Holocene and Neoglacial advances. This paper is concerned with the moraine evolution in the Fiescher Valley, a relatively large glaciated alpine valley system in the Berner Alps, which hosts the fourth-largest glacier in the Alps. A geomorphological and sedimentological analysis, supplied with Schmidt-hammer exposure age-dating, was used on the preserved moraine sequence along a 7-kilometre section of the valley. Calibrated Schmidt-hammer weathering results provide evidence of the multiphase glacier readvances of the Fischer glacier during the Younger Dryas and Early Holocene. The exposure age of Holocene boulders strongly differs with the time of moraine formation as a result of the incorporation of more weathered boulders that originated by earlier glacier fluctuations and rockfall activity. We thus infer that the previously formed Middle Holocene moraines were overridden by the much more extensive LIA advance. Sediment transport pathways in the Fiescher Valley were dominated by subglacial active erosion and transportation pathways of massive crystalline rocks, discernible via the large proportion of subrounded and subangular clasts. We found that significant and multiple glacial remodelling is arguably the most efficient way to reduce the initial platy shape of granitic and gneiss clasts, but a dependence between clast form and roundness with distance is hardly visible.