Valley glaciers and ice sheets

Freshly fallen snow is composed of ice crystals and many air spaces. When you make a snowball, you compress the snow and remove the air spaces. The same happens naturally when snow accumulates; the weight of the snow above compresses the air out of the snow below and converts snow into ice. As the ice becomes thicker it will move down the slope by its own weight. When ice moves it is called a glacier. Glaciers are of two main types.

1. Valley glacier- A moving mass of ice in which the movement is confined within a valley. It begins in an upland area and follows the route of a pre-existing river valley. Today most valley glaciers are found near the tops of young fold mountain ranges, such as the Alps, Andes, Rockies and Himalayas. Examples include the Mer de Glace near the ski resort of Chamonix in the Mont Blanc region of south-east France and the Rhone glacier in south-east Switzerland, the source of one of Europe's largest rivers.

2. Ice sheet- A moving mass of ice which covers the whole of the land surface over a wide area. In some cases the ice is sufficiently thick to blanket the entire area of a continent. In Antarctica, where only the peaks of some high mountains stick through the ice, only a tiny ***** of bare rock is exposed along a few parts of the coast in summer.

There are two main processes (or ways) of glacial erosion.

1. Abrasion- rocks and rock particles embedded in the bottom of the glacier wear away the rocks over which the glacier passes. These sharp-edged pieces of rock of all sizes are held rigid by the ice above and are used as the tools for abrasion. Smaller rock particles have a sandpaper effect on the rocks over which the ice passes, while the sharp edges of the large rocks make deep grooves, called striations.

2. Plucking- this is the tearing away of blocks of rock from the bedrock as the glacier moves. These blocks of rock had been froze to the bottom of the glacier where water had entered joints in the rock and become frozen. The blocks of rock between the joints are pulled away or plucked.

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Glaciers would be less effective at eroding the landscape without the help of freeze-thaw weathering. Before the ice advanced, freeze-thaw left many frost-shattered rocks which were easily removed by the glacier and then used as tools for abrasion. Even when the ice is present, freeze-thaw action affects rocks which outcrop above the surface of the ice because, in a cold climate, there are likely to be many changes of temperature above and below freezing point.

Of the two types of glacier, valley glaciers are considered to be more effective agents of erosion than ice sheets. Confined in a valley, the ice touches the the floor and the sides so that there is more contact between the ice and the rock and therefore more erosion. Also, valley glaciers flow more quickly, partly because of steeper gradients and partly because more meltwater is present to lubricate their flow. There is a plentiful supply of rock fragments from the frost-shattered peaks above, so these glaciers are well supplied with tools for abrasion. However, ice sheets cover and therefore erode a much greater area, so although they erode more slowly, they can still remove a large total amount of rock. As with the other agents of erosion, rocks that are soft or have weaknesses, such as many joints, are eroded more quickly, both by valley glaciers and ice sheets.

If you are sitting north of the line from London to Bristol and take a look out of the window, it must be difficult for you to imagine that just 40 000 years ago all the land you can see would have been part of a snow- and ice- covered white wilderness.

The British Isles were invaded by ice sheets from Scandinavia during the Pleistocene Ice Age, which covered everywhere except for the extreme south of England.

In the higher areas, such as the Cairngorms, the Lake District and Snowdonia, heavy snowfall led to the accumulation of snow and ice in hollows on the rocky mountainsides. These were the sources for valley glaciers which flowed down valleys previously eroded by rivers.

Over the past 10 000 years the world has warmed up. Today no part of the British Isles lies above the snow line (the line above which snow and ice remains all year). However, the present-day landscapes of the British Isles show plenty of signs that ice sheets and glaciers once covered the land.