Abstract
ABSTRACT Boulders in the ablation areas of Alpine valley glaciers were found to not travel along with the ice in a passive manner only. Many show an additional but smaller component of movement towards the south. We investigate this phenomenon and its governing processes using field observations and measurements from terrestrial and aerial photographs of glaciers in the Swiss Alps. We found that large boulders can migrate from their medial moraine due to cyclic formation of classical glacier tables and also a similar process that produces ice tails. The main driving factors behind boulder migration are the size (and shape) of the boulder, ablation, radiation, and surface slope. On glaciers roughly oriented to the east or west, these processes result in a sorting of boulders from the supraglacial moraine towards the southern side, i.e. towards the sun. Future studies complementing our approach using a differential global positioning system should be able to better distinguish between the velocity components of ice flow and boulder migration, determine the precise azimuth of the latter, and investigate the potential influence on photogrammetric feature tracking.
Highlights
Amongst the most prominent surface features of many Alpine valley glaciers are medial moraines and individual boulders of surface debris, many of which form glacier tables
We have shown that existing models of medial moraine development do not fully explain all surface features associated with medial moraines on Alpine valley glaciers
Medial moraines have long been known to grow in width and height relative to the surrounding glacier surface as they travel downglacier, additional effects related to exposure to the sun and ablation need to be taken into account
Summary
Amongst the most prominent surface features of many Alpine valley glaciers are medial moraines and individual boulders of surface debris, many of which form glacier tables. Sometimes artists purposely drew the figures of the staffage too small in order to deliberately emphasize the effect of the phenomenon, in this case the glacier table, which caused as much awe and amazement at the time as it does today (Zumbühl 2009). Glacier tables, impressive ice fronts, waterfalls etc., overtopped by high summits and with tiny figures (e.g. bold alpinists or hunters) in their vicinity, were typical Alpine landscape motifs at the time of the Romantic period (Zumbühl 1980). Glacier tables are often associated with medial moraines. An excellent overview of early relevant literature is given by Eyles and Rogerson (1978a)
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