Through their movement, glaciers not only transport ice but also rock material. The material deposited by glaciers is known as moraines. These typically consist of a densely compacted mixture of clay, silt, sand, and gravel. However, glaciers can also transport massive boulders:
Erratics – also known as erratic boulders – are large rock fragments that are transported by glaciers and deposited far from their origin, often hundreds of kilometers away. These foreign rocks vary in size from a few meters to several cubic meters.
Yeager Rock, a 400-ton boulder on the Waterville Plateau in Washington, was transported by a glacier but is not considered a true erratic, as it is composed of the same rock type as the underlying bedrock. Nevertheless, it vividly illustrates the immense power of glaciers and their ability to transport massive boulders over long distances. (Quelle: https://commons.wikimedia.org/wiki/File:Yeager-Rock-Erractic-PB110039.JPG?uselang=en#Licensing)
Glaciers transport erratic boulders in various ways:
- Englacial transport: Rocks are incorporated into the ice and move with the glacier.
- Subglacial transport: Glaciers break off parts of the underlying bedrock, grinding them down and carrying them along.
- Supraglacial transport: Erratics are carried on the glacier’s surface.
As glaciers melt or retreat, the rock fragments they carry are deposited at new locations. These erratics are striking witnesses to the ice ages. By studying the types of rocks, conclusions can be drawn about the origin of the ice flows that transported them.
Glaciers are constantly moving, transporting erratic boulders over vast distances. These blocks can lie embedded in the ice, on the surface, or at the glacier’s base. As the glacier melts, the erratics are deposited, often hundreds of kilometers from their original location. (Image source: own illustration)
Large Alpine glaciers transported rocks from the Central Alps far northward. In the Achen Valley, you can still find erratics that do not belong to the local geology. Along the Geo.Trail, for example, numerous para- and orthogneisses, foreign to the region and originating from the central Alps, can be discovered.