Glaciers are formed on a shelf or "corrie" where snow
accumulates more quickly than it melts. This is normally on north-facing
slopes and at high altitudes. The increasing mass of snow becomes
more and more compact, eventually turning to ice. Cracks form at the back
of the corrie, and the accumulated ice begins to move over the edge of the
corrie and down the mountainside.
Glaciers move quite slowly - from a few centimeters to several meters per day. Glaciers move in two different ways: flowing and sliding.
Ice can actually flow (slowly). Glaciers are pulled down the mountainside by the force of gravity acting on their huge mass. Glass also behaves like this - an old piece of glass in a window is thicker at the bottom than at the top.
The ice may slide over a layer of water, or a layer of rock debris.
Putting pressure on ice reduces the temperature at which it melts. (Click here to find out about an experiment that shows this.) This means that the base of the glacier may melt, leaving the heavy bulk of ice floating on a thin layer of water. This reduces the friction and allows the ice sheet to move relatively quickly. However, it is still constrained by the rocks in its path, and may stick (refreeze) in places where the pressure is not sufficient.
Just as we would slide down a slope which was covered with loose earth or
gravel, glacial ice can slide down mountainsides where the ground is covered
by material not firmly "stuck down". Again, this allows the glacier
to move much more quickly than ice-flow alone would allow.
When the glacier slides down the mountain, it breaks off large chunks of rock (erosion). The chunks can be up to the size of a house! These boulders join the glacier on its journey down the mountain, which then acts like a giant piece of sandpaper, rubbing off yet more rock (abrasion). This debris (called "moraine") is later deposited by the glacier.
Glacial erosion produces u-shaped valleys, in contrast to the v-shaped valleys caused by rivers. This is because they are very large and relatively inflexible, and can gouge out very large chunks of the mountainside.
The Glacier pushes debris ahead of it's snout. In the summer, when the glacier melts more quickly than it flows, the snout retreats up the mountainside leaving a pile of rubble at the lowest point.
Large 'steps' can be seen along the floor of valleys which were glaciated in the ice age. As the global temperature increased, the snout descended less and less far down the valley each winter, each time leaving terminal moraine at its lowest point.
Debris is deposited along the edges of the glacier. This produces relatively flat, raised ledges along the sides of the valley. In valleys which were once glaciated, these ledges can be used for trainlines, roads and habitation as they are above the flat valley floor and so less susceptible to flooding.
Water on the surface of the glacier flows down through crevasses, to form a river in the heart of the glacier. This river carries small chunks of moraine, and deposits it in a path along the bottom of the glacial valley.
Loess is fine, wind-blown moraine, made up of clay, sand and humus. It forms little hillocks shaped rather like sand dunes . Loess is very fertile, making once-glaciated valleys excellent for farming.
The large boulders carried by the glacier dig into the mountainside and produce scratches down the valley. These scratches are called "striations".
Roche moutonee literally means "sheep rock". These are formed when a
glacier
passes over a hillock. Where the hillock faces the direction of glacial flow
it is smoothed by the "sandpaper" on the base of the glacier. On the leeward
side the glacier bites chunks out of the hillock, producing a rock outcrop
which might be mistaken for a sheep (from a very long way away!).
