Glacial erosion by the Trift glacier (Switzerland): Deciphering the development of riegels, rock basins and gorges

A long-lasting question in glacial geology is how and how fast glaciers were able to shape the distinctive landscapes of the Alps. This study contributes to the understanding on the formation of overdeepened basins, especially the processes and the amount of time involved. We examine the remarkably high (150 m) cross-valley bedrock riegel and the associated overdeepening located in front of the Trift glacier in the central Swiss Alps. A combined approach of field survey with measurements of two cosmogenic nuclides, ¹⁰Be and in-situ ¹⁴C, and a numerical model was used to determine the spatial glacial erosion patterns on the bedrock riegel. Ten samples were taken along two transects; one perpendicular to the glacier flow direction, from outside of the Little Ice Age (LIA) extent down to the centre of the riegel, and the other following the former ice-flow direction across the riegel. Analysis of measured nuclide concentrations shows that the sample outside of the LIA was constantly exposed since the retreat of the Egesen stadial Trift glacier (~11.5 ka). The samples inside the LIA extent indicate a distinct trend of increasing glacial erosion rates from 0 mm/a near the LIA ice margin to high erosion all across the top of the riegel. The resulting minimum glacial erosion rates from samples on the riegel are 0.5–1.1 mm/a (¹⁰Be) and 0.6–>1.8 mm/a (in-situ ¹⁴C) which correspond to minimum erosion depths of 1.6–>3 m (¹⁰Be) and 1–>5 m (¹⁴C). The extremely low nuclide concentrations measured at the riegel highlight the substantial erosion (predominantly abrasion) of the bedrock surface during late Holocene glacier coverage. Field observations suggest that the formation of the overdeepening and, as a consequence, the riegel is due to a combination of valley shape, bedrock structures, glacier confluence and hydrology. We further hypothesise that the gorge is a key factor responsible for this impressive overdeepening, by lowering the threshold for the subglacial meltwater, effectively decoupling the height of the riegel from the depth of the overdeepening.