Glacial Erosion

Glaciers are the best example of mass movement. The work of the glaciers also drifts huge rock material with themselves. The heavy rock material movement also eroded some parts of the bedrocks known as Glacial Erosion. The glaciers cause erosion in three ways: (i) by plucking or quarrying, (ii) by abrasion, and (iii) by frost wedging.

(1). Glacial Plucking/ Quarrying:

While flowing over a jointed rock surface, the glacial ice adheres to blocks of jointed bedrock, pulls them out, and carries them along.

(2). Glacial Abrasion:

The moving ice grinds and polishes the rock surface with the help of rock fragments that are held firmly within the body of the glacier. The abrasion produces striations and grooves on the bedrock surface. A polished surface results when the glaciers perform abrasion by fine silt-sized sediment. The ground-up rock produced by the grinding effect of the glacier is called “rock flour”.

(3). Frost Wedging: 

Thawing and freezing of water in the cracks and joints of rocks break them by wedge action. In this manner rock fragments of all sizes are added to the glacier.

Features of Glacial Erosion:

Striation: Glaciers carry rock fragments firmly embedded in the ice. They scratch, grind or groove the rock surface over which they move. these scratches and grooves left on bedrock and boulders are called “Striations”. The striations indicate the direction of the ice movement.

U-shaped Valleys: Glaciers occupy valleys and flow downhill. As they erode their valleys both laterally and vertically, U-shaped valleys with steep walls and flat floor are produced.

Hanging Valleys: Since the magnitude of glacial erosion depends upon the thickness of ice, main glaciers cut their valleys deeper than those of their tributaries. As a result at the junction where a tributary joins the main glaciers, the floors of their valleys do not meet at the same level. The valley of the tributary stands at a higher elevation than that of the main valley. Such valleys are called “hanging valleys”. When the glaciers disappear, the hanging valleys are occupied by streams, which discharge into the main valley forming waterfalls. Where a valley glacier terminates on land, the streams of melted water flowing on and under the glacier meet downstream to form a single river. The glaciers that end at the sea coast, discharge huge cliffs of ice into the sea. Because ice is less dense than water, it floats. Such floating ice hills are called “Ice Bergs”.

Cirques: The bowl-shaped hollows present at the glacier valley heads in the mountains, are called “cirques”. They are formed mainly by the quarrying and frost-wedging action of the ice. In cirques, a little gap is generally left between the head of the glaciated ice. This gap is known as the “bergshrund”.

Serrate Ridge: As the adjacent cirques, along the opposite side of a mountain are enlarged, the space between them becomes narrow. As a result, sharp divides are formed. Such divides, which have jagged, serrated, and linear crest are called “Serrate ridges”. When three or more cirques surround a mountain summit, a pyramid-like peak is formed. Such a peak is called “horn”.

Roche Moutonnees: These are small mounds of resistant bedrock, which have a typically asymmetrical appearance. The side facing the direction of the ice advance is gentle and smooth, while the leeward is steep and rough. This form results from the plucking action on the leeward side and abrasion on the opposite side.

Fjords/ Fiord: The glaciers that descend from coastal mountains may cut their valleys below sea level. Such valleys produce “fiords”. Fiords are highly over-deepened narrow channels of glacial origin along which the sea encroaches inland. Fjords are found along many coasts including those of Norway, British Columbia, and Alaska.