Glaciers title

In the early 1800s the Swiss naturalist Jean Louis Aggasiz turned the explanation for U shaped deep valleys, scoured rounded features and dropped deposits away from some past flood to glacial action. He did it by studying contemporary glaciers; he realised that there must have been at least one (there have been three or more) ice age.

We now know that in Pleistocene times an ice sheet covered Britain north of London to Bristol, and in the ice age that ended some 25,000 years ago, Scotland, Wales and northern England were covered. These times have given Britain its glacial features.

Similar ice-sheets now only exist in Greenland and Antarctica, and there are smaller glaciers in mountain ranges such as the Alps and the Himalayas. Ice sheets can be extremely thick at Antarctica and Greenland. Ice sheets may travel to the sea to produce icebergs.

The snow line, which varies in height with latitute, is the height at which snow will not completely disappear even in summer. There are hollows, some of which are shaded from the sun, which stay occupied with snow. Water does melt from it, goes into the rock, freezes and thaws to break the rock, and enlarges the hollow. Ice can scrape at the sides and floor, and transport material away. The resultant feature is called a corrie, cwm or cirque and a number turn a mountain top into a horn from a series of aretes. The back wall loses much material and can become steep, and the bottom can be hollowed out to leaver a lake when the ice has gone. The snow itself deepens compacting the lower layers which creates ice, as does the thawing and freezing. When large and heavy enough gravity moves the ice and snow out of the hollow to become a valley-glacier.

glacier downhill

With a push from behind as well as gravity, a glacier can even move uphill a little. It actually moves slower at the sides, scouring away, and slower below than above, and so ice covered valleys all acquire U shapes, due to this wide frozen water action and the moving and dropped rubble beneath the glacier and its tributaries. Winding valleys become straghtened.

Glaciers look after themselves in that moist winds passing over the top condense the water into snow, adding more. Down draughts push away warmer air and so maintain the glacier. The ice that forms melts when under the huge pressure of a glacier (at the bottom it is 28 tons plus per square foot) and this facilitates flow. Also ice crystals crack allowing the ice to roll along.

Differences in speed and sharp drops produce crevices in the ice. These cracks can be dangerous and prevent travel. Snow may cover these cracks which, forming a crust, may or may not hold up a person's weight.

Rock fragments and gravel are called moraine and some fall on top of the glacier. This is because of freeze thaw action on valley sides above the actual glacier forms lateral moraines; where a glacier from a tributary valley joins a valley glacier two lateral moraines combine to produce a medial moraine.

above glacier

end view glacier

The weight and movement of the bigger glacier causes more erosion than a tributary glacier, and thus when the ice is gone a tributary can form a hanging valley with a steep drop down at the old meeting point and a waterfall is left. A glacier at its lower and warmer melting front end drops debris called terminal moraine; the result is erratics or large boulders, and elongated hills called drumlins which block the outflow of the later glacial lake, a lake that is usually very fresh because it is supplied by melting ice from the mountain sides.


Rock-steps are caused both by different rates of erosion by the glacier and by resistance to erosion by rock variations. If a valley floor is overdeepened, for example between two rock steps, a long narrow ribbon lake can form. They can also come from deposits of terminal moraines. Fjords come from the drowning of glaciated valleys often with an overdeepened floor whilst it gets shallow with rock or moraine close to the sea.

Hard rock shows smooth features on the stoss side facing the glacier and rougher plucked features where it goes past on the lee side. This is called a roche moutonee. Hard rock that resists erosion also protects softer rock behind it. This is called a crag and tail.

roche mountonnee

crag and tail

As the glacier goes downhill it can meet a point where the rate of melting balances the rate of movement. This point is the snout of the glacier. Moraine is dropped here, and if the snout moves back and forth, with pauses in between, then there are left a number of minor terminal or recessional moraines.

glacier existence
glacier gone leaving features

When many valley glaciers come on to a lowland plain they can spread out and join into a single ice mass called a piedmont glacier.

In lowlands there is both glacial deposition and erosion, the latter depending on the vigour of the ice sheet. This leaves many rock basin hollows.
A series of low hills come from the terminal moraine of an ice-sheet over huge distances. These are boulder clay areas and can be large erratic boulders or greyish clay or sand. Boulder clay can be shaped into oval drumlins either in relative isolation or in big numbers.
Sometimes stagnant ice is left in boulder clay, and this produces a hollow known as a kettle-hole. River action has filled these lakes in since the Ice Age.
Outwash sands and gravels come from the meltwater streams out of the snout of a glacier and much can deposit when an ice sheet retreats by melting. A winding ridge of sand and gravel is called an esker where streams underneath the ice sheet or beyond drop material.
The wind can blow glacial deposits after the ice sheet had gone. These are called loess deposits.


Examples and Localities

Piedmont glacier Malaspina Glacier, Alaska
Corries with lakes Blea Water Tarn, Lake District
Medial moraines (6) Aletsch Glacier, Alps
Horn/ Pyramidal peak Yr Wyddfa/ Snowdon
Ribbon Lakes Lake Windermere and Coniston Water
Fjord Sogne Fjord: overdeepened to over 1000 yards below sea level
Crag and Tail Edinburgh basalt crag (the castle site) and limestone tail
Rock basin hollow lakes Finnish Lakes Plateau, Baltic Shield
Terminal Moraines North European Plain
Boulder Clay Holderness, East Yorkshire
Drumlins Northern Ireland, Eden Valley
Eskers Finland, north Poland, Sweden
Overflow Channel (water blocked by ice) Newtondale (North York Moors railway route)


Allen, D. L. (Gen. Consultant, with Special Consultants) (1979), Joy of Nature: How to Explore and Enjoy the Fascinating World Around You, London: Reader's Digest Association Limited, 84-87.

Knowles, R. (1976), GCE O-Level Passbook: Geography, London: Intercontinental Book Publications/ Seymour Press, 65-72.


Adrian Worsfold