GLACIAL HEADS AND FLUVIAL TAILS: QUATERNARY LANDSCAPE EVOLUTION IN ALPINE SETTINGS

Glaciers are efficient erosional agents. Their fluctuations during the Quaternary modified alpine headwaters, leading to centurial-scale pulses in sediment supply to the fluvial system. Because glaciers only erode when sliding against bedrock, understanding what governs sliding rates is key to understanding erosion history in glaciated basins. These controls include the thermal state of basal ice and daily-to-seasonal variations in glacial hydrology. We briefly review glacial erosion mechanics, and explore the controls and consequences of glacial erosion using numerical models of landscape evolution. At short length scales, glaciers generate rock bedforms such as roche moutonnée that reflect competition between the two major erosion processes of abrasion and quarrying. At the valley scale, steepened headwalls, flattened-to-overdeepened valley floors, and hanging valleys are an inevitable consequence of glacial erosion.

On million year time scales glaciers can erode their valleys sufficiently to alter the length of subsequent glaciers. This explains widespread observations of “far-flung” early glaciation moraines that are substantially outboard of LGM moraines. In addition, the lateral migration of steep walls generated by differential glacial erosion further modifies the hypsometry of the landscape on which subsequent glaciers form. During interglacials, sediment in the talus and rock glaciers at the bases of these migrating walls is effectively stored in alpine valleys until the next glacial.

Glaciers also respond to interannual variations in air temperature and precipitation. This climate noise can cause kilometer-scale glacial length fluctuations even in steady-state climates, which in turn generates terminal moraine complexes, complicates the conventional inference of paleoclimate from glacial moraines, and influences the temporal pattern of erosion.

Strong pulsing of sediment output from glacial valleys on glacial-interglacial timescales governs the evolution of downstream fluvial systems, leading to incision and aggradation of the fluvial channel that are temporally linked to fluctuations in glacial sediment output. In hard bedrock valleys, variation in sediment supply results in alluvial terraces, while in more easily eroded rock, strath terraces are formed.