Ice and glacial processes

For much of the last 2.6 million years, the Earth's climate has swung between cold glacial periods and warmer interglacials. In the cold periods, ice sheets and glaciers covered much of the UK.

On a map of the last ice age, the ice reached its greatest extent about 27,000 years ago:

• an ice sheet covered almost all of Scotland, Wales, northern England and Ireland;
• in the west, it reached south to the Bristol Channel; in the east, lobes of ice pushed down the east coast of England;
• most of southern England stayed free of ice, but was frozen tundra.

The ice then melted back, and the uplands were left with the landforms it had carved.

• Freeze-thaw weathering: water in cracks in the rock freezes and expands, widening the cracks. Repeated freezing and thawing breaks off pieces of rock, which fall onto the glacier or pile up as scree.
• Plucking: meltwater at the base of the glacier freezes onto the rock. As the ice moves, it pulls away pieces of rock, leaving a jagged surface.
• Abrasion: rocks frozen into the base of the glacier scrape the rock beneath like sandpaper, leaving it smooth, with scratches called striations.

Glaciers move slowly downhill under their own weight, helped by meltwater at their base, which lets them slide.

• Rotational slip: in a hollow high on a mountain, the ice moves in a curve, rotating as it slides. This deepens the hollow, and helps form a corrie.
• Bulldozing: the front of the glacier, its snout, pushes loose rock and soil ahead of it, like a bulldozer.

As a glacier moves, it carries huge amounts of rock: on its surface, inside it and beneath it.

When a glacier melts, it deposits the rock it carries.

• Till is material dropped by the ice itself. It is unsorted: a mix of clay, sand and boulders, with sharp, angular pieces, as the ice drops everything at once.
• Outwash is material carried away by meltwater streams. It is sorted, with the heaviest material dropped first, and its pieces are rounded by the water.