Glacial Action – SS2 Geography Lesson Note (First Term)

Glaciers are an important agent of erosion, transportation and deposition in temperate and mountainous regions. A glacier is a large mass of ice in motion. Glaciers erode by sapping, plucking and abrasion, and leave deposits such as boulder clay, erratics, drumlins, eskers, terminal moraines and outwash plains.

Behavioural objectives

By the end of the lesson, students should be able to:

• define ice, snow, snow-line, glacier, glaciation and snow field;
• explain sapping, plucking and abrasion as actions of glacial erosion;
• name the features of glacial deposition in lowland areas;
• describe each of these features briefly.

What are the terms associated with glaciers?

1. Ice: a solid form of water which results from freezing when the temperature is below 0°C.
2. Snow: frozen water vapour that falls in the form of crystals from the atmosphere.
3. Snow-line: the lower limit of snow cover on a mountain.
4. Glacier: moving ice, that is, a large accumulation of ice in motion.
5. Glaciation: the wearing away of the earth’s surface by glaciers.
6. Snow field: the region or area that is permanently covered by snow.

How do glaciers erode the land?

1. Sapping: the breaking up of rock by alternate freezing and thawing of water at the bottom of cracks.
2. Plucking: the tearing away of rocks which have become frozen on the side or bottom of a glacier.
3. Abrasion: the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.

What features does glacial deposition form in lowland areas?

1. Boulder clay: stones of various sizes in a mass of sand and clay.
2. Erratics: transported rock fragments which are composed of materials entirely different from the bedrock of the region where they are deposited. They are deposited when the ice carrying them melts into water.
3. Drumlins: features composed mainly of boulder clay.
4. Eskers: long, narrow ridges of sand and gravel deposited by melting water. They are usually porous.
5. Terminal moraines: features made up of boulders which are deposited at the edge of the ice sheet.
6. Outwash plain: a large area of sand and gravel that is washed down from the mountainous zone and deposited beyond the terminal moraine.

Evaluation

1. Define a glacier and a snow field.
2. Explain the terms sapping, plucking and abrasion.
3. What are erratics, and how are they deposited?
4. Describe an esker.
5. Where does an outwash plain lie in relation to the terminal moraine?

Assignment

1. List the three actions of glacial erosion and explain each one.
2. Name and describe any three features of glacial deposition in lowland areas.