Water

A large conveyor belt

Glacier movement, friction with the rock along the sides and on the bottom, falling material from the surrounding sides, dust brought by wind, animal carcasses, waste left by man, including war relics and the bodies of unfortunate soldiers or mountain climbers : everything can be “captured” and swallowed by the ice of a glacier, on the surface or within. The movement of a glacier, described before as a sliding downstream movement, is actually more complex and it contributes to letting debris penetrate deeply in the accumulation area, and to bringing them out in the ablation area, where also melting process collaborates in uncovering them. When a glacier moves forward, like a gigantic scraper, it pushes incoherent debris and rocks below and in front of itself; when it retreats, it abandons all the material it was carrying, forming glacial deposits also known as till, from a Scottish term. These deposits have different names, depending on how the glacier arranged them (e.g. “spreading” them on the sides and on the bottom, crushing and pressing them with its weight, as in the case of “lodgement tills”, or accumulating them as the ice containing them slowly melts, as in the case of “ablation tills”). Ice deposits have unique unmistakable characteristics (for example the contemporary presence of large size blocks, very coarse particles and a very fine matrix, rounded gravel that is striped due to the enormous pressures and the reciprocal friction it is subjected to during transportation), so that it is easy to recognise them even when the ice that produced them disappeared a long time ago. Thanks to the discovery of deposits of glacial origin, it is possible to reconstruct the succession of various glaciation episodes over the years : at times the locations in which ice deposits have been found were truly extraordinary, as for example, recent findings in the desert in Namibia or in the Sahara desert, proofs of a glaciation of over 400 million years ago. Younger deposits often also mantain particular features, as moraines, which enable the reconstruction not only of the presence of ice, but also the shape of the front and the height of the sides. Glaciers can be tens or hundreds of kilometres long, in the past there were even more wide-ranging ice bodies. Also the study of the type of rocks that form the ice deposits provides important information, enabling us to reconstruct the course of ancient glaciers that are now extinct. For example, in the territories to the north of Milan, there are rock cobbles originating from the Valtellina and Valchiavenna valleys, which enables the reconstruction of the course of an ancient glacier that in various spurts, flowed down the valley of the Adda river, occupying the area that now is Lake Como. Detailed studies in this area, enabled the reconstruction of the exact course of the various tongues the main glacier divided into, avoiding various nunataks, till it reached the plain. The study of glacier sediments is therefore fundamental in order to understand the variations in the expanse and shape of glaciers in the past, but this can be done in an efficient manner only if the processes regulating the behaviour of glaciers today are known very well.