GLACIATION

GLACIATION

 Glaciation refers to the process whereby a certain area on the earth’s surface is affected by glaciers (moving ice). Glaciation also refers to the process that takes place due to the influence of moving ice.

Glacial erosion

Glacial erosion, which predominates in the highlands, consists of the following mechanism or processes:

Sapping: This refers to the breaking up of rocks by alternate freezing and thawing of water at the bottom of cracks between a mass of ice and the side and floor of a valley, or the side of a mountain.

Plucking: This is the tearing away of the blocks of rock which have been frozen into the sides or bottom of a glacier. 

Abrasion: This is the wearing away of rocks beneath a glacier by the scouring (scrapping) action of the rocks embedded in the glacier. 

Feature produced by glacial erosion

i. Cirque (corrie): A semi-circular, steep-sided basin cut into the side of a mountain, or at the head of a valley. It is formed by the process of plucking, which steepens the basin, and abrasion, which deepens the valley. Some corries contain glaciers, but in others the glaciers have melted and they now contain lakes (sometimes called tarns). Examples of tarns are Lake Tana in Ethiopia and Teleki tarn on Mount Kenya.

ii. Arete: A steep-sided, knife-edged ridge separating two cirques. It is formed by the cutting back of the walls of cirques by plucking. Examples of Artes are found on Mount Kenya.

iii. Pyramidal peak: A jagged peak with a steep sided, angular horn. It is formed by the steepening of the back walls of several cirques which lie on the sides of a mountain. Examples of pyramidal peaks are found on Mount Elgon.

iv. U-shaped valley: A steep-sided, flat-bottomed, wide valley which contains features formed by both glacial erosion and deposition on the foot of the glaciated highland. It is formed by vertical and lateral erosion of moving ice. Most U-shaped valleys were originally river valleys.

v. Hanging valley: A tributary valley of a U-shaped valley which ends abruptly, high above the floor of the U-shaped valley and separated from it by an almost vertical slope. It is formed due to unequal down-cutting on the tributary valley.

vi. Rock basin: An irregular depression in the floor of a U-shaped valley formed by unequal glacial erosion of the bedrock. It develops when the thickness and weight of a glacier increase, e.g. at the junction of two glaciers. Sometimes a rock basin becomes a lake when the glacier melts.

vii. Truncated spurs: These are blunt-ended rock ridges which descend from the steep sides of a U-shaped valley or glacial trough. They are often separated by hanging valleys.

viii. Ice-eroded plain: An extensive area once covered by an ice sheet which smoothed off the original landforms to give a rounded topography, with large areas of bare rock scratched by boulders embedded in the base of the ice, and rock basins in areas of weak rock, and the whole swept almost clean of the original weathered rock.

ix. Roche Moutonnee: An outcrop of resistant rock smoothed by a glacier on the upstream side into a gentle slope. On the downstream side, the glacier erodes by plucking to give steep and jagged slope. It is formed where resistant rocks rise above the surrounding land surface. The upstream side of the rock is plucked to a steep slope.

x. Crag and tail: A head of resistant rock which protected a weaker rock from ice erosion on the downstream side.

Depositional features of glaciations

Moraine: These are unsorted rock fragments of all sizes, from sand to boulders, formed partly by frost action and partly by glacial abrasion, transported by a glacier and dumped in ridges or sheets.

Types of moraines

A moraine that forms along the sides of a glacier is called lateral moraine; that along the front of the glacier is called terminal moraine, and that at the bottom of the glacier is called ground moraine. When two glaciers join, their inner lateral moraines join together and give a medial moraine.

Boulder clay plain: A plain made of clay and boulders, deposited by ice sheets and glaciers over a surface. 

Drumlin: Elongated, oval-shaped hill made of boulder clay and about 1 km long and 25 to 100 m wide. Each drumlin is a small hill, tending towards an egg shape, with its steepest slopes and summit at the up-ice end.  

Eskers: An esker is a long, narrow, winding ridge of stratified sand and gravel. The materials that form an esker are deposited by sub-glacier streams which retreat their way in the channel under the ice. Eskers reach up to 40 metres high. They are mostly found in Scandinavian countries.

Kame: Is an irregular-shaped mass of stratified material formed as a delta on the surface of a stationary glacier or at its margin. It is a mound-like hill of poorly sorted material mostly sand and gravel, deposited at or near the terminus of a glacier.  

Erratic: A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. Most erratics can be found at Kimberlay (South Africa), North East USA and Wales in Britain.