Subglacial plougging and drainage patterns in the glaciated valley of Andorra (SE Pyrenees), dynamics and rheology

Abstract

The Alps were covered by a complex ice sheet comprising various centres of glaciation with occasionally dome-like structures from which huge valley glaciers flowed out. Systems of transection (interconnected valley glaciers) as well characterised the Alps. The equilibrium line altitude (ELA) across the Alps was between 1000 and 1700 m, with a general trend of rising from the margins to the centre. At sites farther to the east and south, smaller glaciers were present. Glaciers extended onto the northern and southern forelands and formed huge piedmont lobes. Frontal moraines filled the amphitheatres broadly defined by moraines built-up during earlier glaciations. Fields of drumlins were shaped in the footprints of the large piedmont glaciers on the northern flank of the Alps from the Rhone Glacier in the west to the Salzach Glacier in the east. The glaciers delivered remarkable volumes of outwash. On the south side of the Alps, huge fans were constructed. In the north, outwash gravels were largely confined to the established river valleys. The timing of the Last Glacial Maximum Alpine glacier fluctuations has been studied with radiocarbon dating, luminescence techniques, and cosmogenic nuclide surface exposure dating. Expansion of glaciers can be shown to have begun as early as 32.5–30.5 ka cal BP, with glaciers reaching the forelands already by 29 ka cal BP. Most systems exhibit two main advance phases, which in the south have been dated to 26–25 and 23–22 ka cal BP.