school energy&environment

Glacial landscape

A glacier is a moving mass of ice. This movement has an erosive action that shapes the Earth’s surface in cold regions. In the history of the Earth, during the Quaternary period, almost a fourth of the lands that had emerged from the sea were occupied by icecaps. Icecaps stretched to the northern regions of America, Europe and Asia that today are characterized by a mild climate. They were thousand of metres thick. When glaciers started to move forward, they deeply modified the land surface, changed river flows, stopped vegetation growth and forced animals to withdraw towards southern regions. During the hottest periods, the ice withdrew to the north, leaving deposits of the materials that they were transporting on the land. The regions that got free from the ice were now covered by forests and populated by animals, but kept the traces of erosion and accumulation of glacier materials.
Examples of erosion and accumulation and glacial landscapes can be seen nowadays on the Alps and the Himalayas.
Glacial erosion
The downward movement of a glacier acts on the rocks that compose the land as if it was a bulldozer: it collects and transports various blocks, of different sizes. At the end of a glacier there can be a watercourse, which exercises an erosive action on the underlying rocks, like any other river that runs on the surface.
The result, once the glacier has withdrawn, are smooth rocks and cracks and grooves on the rocks. The land irregularities are reduced and the rocks look like flat humps: roche mountonnée. By observing these rocks geologists can re-build the history of the territory, since according to the hump direction it is possible to understand the direction the glacier moved to.
In an Alpine landscape, the main and the secondary valleys are shaped by the glaciers action that has eroded the valleys and the mountainsides. These valleys have a U-shape, while those valleys that have been created exclusively by the action of a watercourse or a river are narrow and have a V-shape. Glacial valleys, on the top, have a semicircular shape, which is occupied by a small glacier or a small lake surrounded by steep rocky walls. This is the glacial cirque, the place where the snow piles up. That snow is later transformed into the ice that feeds the glacier. When the depositing snow is more than the snow that melts during the hot periods, the glacier grows and moves towards the valley.
Some typical glacial valleys were invaded, after their creation, by ocean waters. They are called fjords (they are typical of Norwegian coasts). Fjords are U-shaped valleys that were carved by the glaciers that had run down from the nearby mountains during the ice periods. During those periods, the sea level was lower than today. A big quantity of water at that time existed as ice. The following ice melting provoked an increase in sea level. The water invaded the valleys near the coasts.
Types of deposits: moraines
When the ice of a glacier melts and disappears, it leaves the transported rocky material on the ground and forms:

• moraines, formed by the debris coming from the glacier surface;
• floor moraines, formed by the debris that came from the glacier floor;
• erratic blocks, that are very big and weigh some tons. They are transported for hundreds of kilometres and left on completely different rocks. When geologists find a rock that is completely different from the near ones, they understand that in ancient times that rock was transported and deposited by a glacier.

The information that can be conveyed by moraines and erratic blocks is very important, as it helps us reconstruct the events and the climate of past geological periods. Moraines indicate the shape, help us reconstruct the movements and the maximum dimension reached by the glacier. It is very important to study hills and small moraines that are present in the Plain of the Po, as they show that the area was covered by icecaps.