Water

Who went this way?

All glaciers leave traces of their passage, traces that may remain even for thousands or millions of years. By stuying present day glaciers, geologists are able to easily recognize the evidence of the existence of ancient glaciers. A glacier, moving and sliding on the bedrock leaves two different types of traces : it may deposit material carryed inside and on its surface, thus giving rise to ice deposits accumulated in characteristic features that are easy to recognize, or it may erode the rocks it is moving on, leaving smooth and polished surfaces. Ice deposits are generally characterized by a granulometry that includes very fine material deriving from crushed grains and debris due to friction with other grains and with the bedrock, and also very coarse material, including blocks that are several metres in diameter. This derives from the viscous property of ice, due to which a load of materials of very different weights and densities can be taken up, unlike other agents of transportation that are much more selective, (as for example the wind, that can only transport sand, or running water that can transport materials of different sizes depending on the speed of the current). The larger boulders are known as “glacial erratics”, and are a good method for understanding up to what heights and for what distance the glaciers of the past moved. If the morphology of ice deposits is preserved, this also provides evidence regarding the shape of the front, the characteristics of the transportation and movement of the glacier, its advance and retreat, and many other aspects. The more characteristic and well known forms are surely moraines, which can be lateral moraines, on the sides, formed between the glacier edge and the slope, or frontal or terminal moraines, which are deposited in front of the glacier, generally forming concentric arcs. Other less known forms in the Alpine glaciers originate at the base of the glacier, due to the effect of deformations caused by the weight of the ice, as in the case of fluted moraines or drumlins, and due to the effect of the water circulating at the base, as in the case of eskers. Other forms originate from the contact of the glacier and the slopes; in ice contact deposits like kame terraces, depressions are formed between the lateral moraine and the slope, which can lodge small lakes and are filled with debris coming from the slopes, such as landslides or avalanches deposits. The study of deposits and morphology enables a detailed reconstruction of the morphology and characteristics of ancient glaciers and is fundamental in order to reconstruct environments and climates of the past. Erosion or exaration features are also excellent evidence of the passage of a glacier and can at times be the only type of trace remaining. These can be very large scale features, as entire valleys with the characteristic U shaped profile, ice cirques separated by narrow crests (forming the so-called glacial horn, as in the case of the Matterhorn), or they can be seen in the form of specific rocky protrusions, the roches moutonnées also known as whale-back rocks, due to the elongated and rounded form. Roches moutonnées are smooth and polished due to the abrasive action of the ice containg a lot of debris, and they are often characterised by stripes and grooves in the rock due to a scraping process on the bedrock, and these enable to reconstruct both the passage and the direction of the glacier flow. By the study of morphology and deposits left by the glaciers, the reconstruction of the maximum limit reached by glaciers during the Quaternary glaciations, is very important. The acronym MEG (Maximum Extension Glacier) indicates the maximum height reached by Pliocene and Quaternary glaciers, while the term LGM (Last Glacial Maximum) indicates the maximum height reached by the glaciers during the last glaciation : the two levels are not the same, specially in plains, as the glaciers did not reach their maximum expansion during the last glaciation. The age of the more recent glacial deposits has been calculated by observing the state of weathering of the rocks forming them, the level of soil development , that determines a different amount of vegetation, the age of vegetation (dendrochronology) and lichens (lichenometry) covering the rocks.