• Every 200-250 million years, climatic activity leads to falls in temperature of over 5 degrees centigrade. These periods of great ice activity are known as Ice Ages.
• The most recent is the Pleistocene period of the Quaternary era, which began 2 million years ago. The earth only became warmer during the last 20,000 years.

  Reasons for fall in Earth's temperatures

• Movement of tectonic plates leading to an increase in the altitude at constructive margins.
• Changes in ocean currents
• Variations in sunspots changing the amount of radiation received by the earth
• Changes in level of atmospheric CO2.

  Glaciation

• Glaciation is the work of ice in a landscape.
• A glacier is a mass of moving ice confined in a valley.
• When air temperature falls to sub-zero, water vapor condenses and freezes to form snow.
• If the snow doesn't melt, a permanent snow covering is formed known as a snow line.
• If the accumulation of snow continues, the snow hardens into ice crystals separated by air spaces. Due to pressure, ice melts and water flows into the air spaces and re-freezes. This turns the snow into a granular mass known as a ne've or firn.
• Finally, all the air spaces disappear and a glacier is formed. Ice blocks may break off to form icebergs.

  Ice Movement

• Ice may move through one of these three processes:

1. Plastic Flowage: Ice has plastic qualities and may flow en-masse like a viscous liquid
2. Basal Slip: The process through which ice slips and slides over the underlying rock.
3. Internal Shearing: Movement similar to rock faulting involving differential sliding along planes.

• The speed of ice movement depends on the gradient of the slope and the thickness of the ice.

  Ice Erosion

• This involves three processes:

1. Plucking: Parts of the underlying rock are frozen into the base of the ice and pulled away.
2. Glacial Abrasion: The grinding process where stones frozen in the ice scrape and scratch against the underlying rock.
3. Sapping: The breaking of rocks by the alternate freezing and thawing of the ice at the bottom or sides of the mass.

Landforms of Glaciation

1. Cirques: Steep sided rock basins semi-circular in plan. Could develop into corrie lakes called Tarns.
2. Aretes: Steep sided knife edged ridge separating two cirques.
3. Pyramidal Peak: Jagged peak formed by the steepening of the back walls of several cirques on the sides of a mountain. Also called horns.
4. U-Shaped Valley: A broad flat-bottomed steep-sided valley with a U shape. Also called a glacial trough.
5. Fjord: A deep narrow arm of the sea with steep sided parallel walls.
6. Hanging Valley: A tributary of a U-Shaped Valley ending abruptly high above the floor of the U shaped valley and separated from it by an almost vertical slope.

Erosional Landforms

1. Rouche Moutonee: An outcrop of resistant rock smoothed by ice on the upstream into a gentle slope and plucked on the downstream end to give a steep jagged edge.
2. Crag and Tail: A knob of resistant rock (crag) which protects a weaker rock (tail) from ice erosion on the downstream slope.
3. Ice-Eroded Plain: An extensive area once covered by an ice sheet which smoothed off the original landforms to give a rounded topography.

Depositional Landforms

Those formed by unsorted materials:

1. Boulder Clay Plain: A monotonous hummocky plain made of clay and boulders and deposited haphazardly by ice sheets over a surface.
2. Drumlins: Elongated ovoid low hummock made of boulder clay.
3. Terminal Moraine: Ridge like feature made of boulder clay marking the edges of the ice.

Those formed by sorted fluvio-glacial materials:

1. Esker: Steep sided ridge made of gravel and sand.
2. Kame: An irregular shaped mass of stratified material formed as a delta on the surface of a stationery glacier.
3. Outwash: Made of gravel and sand and developed outside of the terminal moraine by melt waters from the ice depositing sorted materials.