Short essay on the Important Features of Erosion

(i) Striations:

The slowly moving ice scraped and ground away much solid bed-rock. Left behind were smoothly rounded rock- masses bearing countless minute abrasion marks, scratches, called striations.

(ii) Roches moutonuees:

They consist of asymmetrical mounds of rock of varying size, with a gradual smooth abraded slope on one side and a steeper rougher slope on the other. The ‘stoss side’, i.e., the side from which the ice was approaching is characteristically smoothly rounded end the other side, i.e., the ‘lee side’ where the ice plucked out angular joint blocks, is irregular and blocky. They are also known as ‘sheep-rocks’.

(iii) Crag and tail:

Sometimes very hard rocks like volcanic plugs offer great resistance to the ice-flow and stand as pillars in the glaciated valley. These structures are called crags and the lee side which is sloping in this case is the tail.

Depositional features:

The term glacial drift includes all varieties of rock debris deposited in close association with glaciers. These deposits may be classified into two groups:

(1) Stratified drift, consists of layers of sorted and stratified clays, silts, sands etc. deposited by the melt-water streams and are also known as Glacio-fluvial deposits.

(2) Till:

It is a heterogeneous mixture of rock fragments rang­ing in size from clay to boulder which are unsorted and unstratified. A consolidated till is called ’tillite’. The various depositional features are as follows:

(a) Drumlin:

It consists of glacial till, which is a low mound of clay containing cores of bed-rocks. Uphill sides are blunt and down-hill sides are smooth and gently sloping. The long ax of each drumlin parallels the direction of ice-movement and thus serves as indicators of direction of ice movement.

(b) Basket of egg-topography:

The drumlins commonly occur in groups of swarms, which may number in the hundreds; the topo­graphy produced by them is peculiar and is known as basket of egg- topography.

(c) Ground moraine:

Between moraines, the surface over-ridden by the ice is overspread by a cover of glacial till, known as ground moraine. Thus it is the sheet of debris left after a steady retreat of ice.

Glacio-FIuvial Deposits:

(i) Outwash plain:

It is also known as over wash plain. Glacial streams carry a huge quantity of rock-debris and then form fan-like plains beyond the terminus of glaciers. These are stratified. When they occur on valley floors such outwash plains are called ^‘valley trains’.

(ii) Karnes or kame terraces:

These are formed on the top surface of a glacier where the surficial melt-waters wash sediments from the top into depressions. As the ice melts the material that formerly fiiled depressions on top of the glacier is dropped and makes small hills, which are more or less flat-topped and are known as kames. Terraces, called kame terraces, are built in this way.

(iii) Eskers:

These are winding steep-sided ridge-like features built of stream borne drift. These are also known as Osser or Oss.

(iv) Erratic:

These are stray boulders of rocks which have undergone a prolonged glacial transport and have subsequently been deposited in an area, where the country rocks are of distinctly different types. At times they are delicately balanced upon glaciated bed rock, and are called poking or logging-stone.

(v) Kettles:

Drifts occurring in the vicinity of a glacier and particularly those lying near about the ice-terminus are ordinarily found to contain a number of depressions, some of which may give ties to lakes or swamps. Such hollows are known as kettles.

(vi) Varves:

These are layered clays alternating with coarser and finer sediments.