The erosive action of glaciers [ENG]
Glaciers are among the most effective agents in modifying the alpine landscape: they can erode the underlying substrate and transport eroded materials and/or those that have fallen onto the glacier over great distances.
The forms of erosion depend on the temperature at the glacier-substrate interface (i.e., the area where the ice ends and the underlying ground begins), the amount of debris within the glacier, and the hardness of the substrate. In the presence of ‘temperate’ glaciers (with a basal temperature close to the melting point and thus with water at the glacier-substrate interface), the substrate can be eroded in various ways and at different scales.
Erosion forms on a metric-hectometric scale
At this scale, the rock beneath the glacier is smoothed upstream, fractured, and removed downstream, creating what are known as ‘roches moutonnées' (or sheepback).
In practice, the moving glacier can erode the ground down to the bedrock, resulting in rock formations with an asymmetrical structure: rounded on the upstream side, which bears the pressure of the moving ice, and typically tapering and irregularly thinning downstream, where the glacier’s push weakens. These effects can extend across scales ranging from one meter to several hundred meters.
Glacial Cirques consist of large depressions carved into mountain slopes and enclosed by a rocky threshold, generally smoothed or formed by roches moutonnées with the truncated side facing downhill.
Tors, on the other hand, are residual landforms that were never covered by glaciers. The minimum elevation at which Tors, and generally sharp, jagged ridges, are found indicates the ‘trim line,’ which marks the maximum altitude ever reached by glaciers.
The trim line here in the Passo Gavia area is between 3000 and 3050 meters, showing that even during the maximum Pleistocene glaciation, a true ice cap did not exist. Instead, there was a system of large valley and piedmont glaciers with numerous "nunataks" (mountain peaks not covered by ice) and Tors outside the glacial mass.
Erosion forms on a metric-centimetric scale
Typically, other erosional features can be found on the surface of roches moutonnées, such as ‘glacial striations’ or ‘crescentic gouges.’
Glacial striations are U-shaped grooves parallel to the direction of glacier flow and sometimes show a gradual decrease in depth along the flow direction. These grooves are formed by the abrasion caused by a ‘clast’ (a fragment of another rock) embedded in the glacier, which erodes the underlying rock until it is worn down to dust.
Crescentic gouges, on the other hand, are small, arc-shaped depressions with the convex side facing in the direction of glacier flow, caused by the removal of rock fragments from the substrate.
If you find yourself at the coordinates listed, you are on the Sforzellina Glacier Nature Trail, near the Gavia Pass, in the Stelvio National Park. The trail was inaugurated on August 21, 2010, with the aim of showcasing the geological features of the high-altitude area surrounding and bordering a glacier. Examples of what erosion has done to the rocks of the substrate are all around you.
To log this cache, you must answer the following questions via messages on Geocaching.com:
1 - What type of rock is exposed everywhere?
2 - How is a ‘roche moutonnée’ formed?
3 - In what direction do the ‘glacial striations’ on the rocks around you run? (e.g., 210°, 90° relative to North)
4 - Attach a photo to the log showing you or something that represents you.