Ice sheets once covered huge areas of both the Northern and Southern hemispheres. Today they have retreated and are restricted to Polar Regions and the highest mountain areas, but the landscapes carved during those earlier icy times still remain.
We tend to regard ice as a solid yet, under pressure, it can flow in the same sort of way that rocks flow within the Earth’s mantle. The structure of ice is very similar to that of rock, too. As snow compacts, air is squeezed out and it slowly turns from a white crumbly texture into a blue crystalline substance, the crystals being of ice rather than minerals. They are lubricated by a microscopic film of water that is kept as a liquid by dissolved salts.
Ice can truly transform the Earth’s landscape. It occupies a greater volume than water so, as it freezes in crevices and joints, it acts like a wedge, gradually breaking off pieces of rock or even boulders. Meanwhile, snow accumulates around the mountain peaks, either triggering avalanches or compressing into ice.
Eventually, the ice begins to flow, making a scooping action as it starts to move down the mountain side. At very high latitudes ice covers everything in a sheet that may be hundreds of meters thick. Within it, there may be faster flowing ice streams as the ground underneath falls away, is lubricated by mud or is even warmed by volcanic activity. as it thins, the sheet can part around rocky outcrops, or nunataks, to form valley glaciers and then reunite on the other side into what are called piedmont glaciers.
The flow rate of ice sheets and glaciers can be very slow- between a few and a few hundred meters a year. So, to maintain the same flow as even a small mountain stream, a glacier has to fill the whole valley. As it goes, it grinds the rocks underneath it into fine flour, and boulders embedded within its deep striations in the sides of the stepped valley that is being carved by it.