Davis suggested that a glacial topography is a climatic accident that happens to normal cycle of erosion, i.e., climate gets very cold and the river freezes. Instead of rivers of water there are rivers of ice, called glaciers, operating as the main geomorphic agent. The geological action of glaciers, ie., erosion, transportation and deposition together constitute, what is known as glaciation.
Formation of glaciers
Under the influence of pressure and moisture, the snow flakes change into a granular ice mass, known as néve in French and ‘firn’ in German. When the ice becomes so thick that the lower layers become plastic, outward or downhill flow commences and an active glacier comes into being.
Type of glaciers
There are three major types of glaciers :
(i) The valley glaciers (Mountain glaciers or Alpine glaciers).
(ii) Piedmont glacier - these are intermediate in form and origin, between valley glacier and ice sheet. They are formed at the foot of the mountains.
(iii) Ice sheet - These are huge covers. of ice and are also known as ‘Continental glacier’.
Accordingly, there are two types of glacial topography, one related to the valley glaciation and the other to the continental glaciation.
Topography of valley-glaciation
There are two sets of features resulting from glacial erosion and glacial deposition, (since the rock wastes and other loads are carried frozen within the ice during their transportation, in case of glaciers).
Erosion - Erosion by glaciers take place due to
(i) plucking,
(ii) rasping and
(iii) avalanching
Plucking is also known as frost-wedging or quarrying. During the Summer months the surface parts of a glacier may partially melt, This melt water or rain water seeps down along the sides of the ice mass, finding its way into the cracks and fractures in the rocks along the edges and at the head of the glacier. At night or when the temperatures drop this water freezes. It breaks up the rock by frost-action, and with the movement of the glacier they are frozen in suspension in the ice.
Rasping is the term used to describe the scraping or abrasion by glacial action. The front edge of glaciers function as bulldozer, pushing and scraping the ground in front of the ice.
Avalanching is the process of mass-wasting. Along the margins of a valley glacier the valley sides are scrapped and blocks are broken off which become frozen into the ice and are carried away. This leads to under cutting of the sides of the valley and pave the ground for slumping, sliding and debris avalanching which bring great quantities of rock-waste on to the top surface of the glacier.
Features of Erosion
1. Cirques. These are circular depressions formed by plucking and grinding on the upper parts of the mountain slopes. These are also known as ‘corries or amphitheatre’.
2. Arete. This name is applied to the sharp ridges produced by glacial erosion. Where two cirque walls intersect from opposite sides, a jagged, knife like ridge, called an ‘arete’ results. It is also known as ‘comb’ or ‘serrate-ridge’.
3. Horn. Where three or more cirques grow together, 3 sharp pointed peak is formed by the intersection of the aretes. Such peaks are known as 'horns'.
4. Col. Where opposed cirques have intersected deeply, & pass or notch, called a ‘col’ is formed.
5. Gitacial-trough. Glacier flow constantly deepens and widens its channel so that after the ice has finally disappeared there remains a deep, steep walled, U-shaped valley, known as glacial trough.
6. Hanging valley. Tributary glaciers also carve ‘U’-shaped troughs. But they are smaller in cross-section, with floors lying high above the floor-level of the main-trough, i.e., main glacial valley. Such valleys are called hanging-valleys.
7. Fiords. When the floor of a glacial trough open to the sea lies below sea-level, the sea-water will enter as the ice-front recedes, producing a narrow estuary, known as ford or fjcrds.
8. Tarns. The bed-rock is not always evenly excavate under a glacier, so that floors of troughs and cirques may contain tock-basin and rock-steps. Cirques and upper parts of troughs thus are occupied by small lakes, called tarns.
Depositional features of valley glaciers
Deposition by a glacier takes place when the ice begins to melt and the glacier slows down and vanishes, losing its transporting power. The unstratified, unsorted debris dropped more or less in a random fashion by glaciers form deposits known as morains. Three types of moraines are known, lateral, medial or median, and terminal. These three types are differentiated on the basis of their location in the valley.
(a) Lateral-moraine. Deposits of ridge-like pattern formed along the margins of the glaciated valley are known as lateral moraines.
(b) Medial-moraine. It results due to coalescence of two lateral moraines, where two ice streams join.
(c) Terminal-moraine. These are accumulation of rock-debris at the terminus of a glacier.
(d) Recessicnal moraine. Where glacier retreats in a halting manner, a series of concentric moraines is formed, known as ‘recessional moraines’.
Topography of ‘continental glaciation’. Like valley glaciers, the continental glaciers proved to be highly effective eroding agent. But continental glacier erodes only by plucking and rasping methods, but erosion process, like avalanching is absent in case of continental glaciers.
Features of Erosion
(i) Striations. The slowly moving ice scraped and ground away much solid bed-rock. Left behind were smoothly rounded rockmasses bearing countless minute abrasion marks, scratches, called striations.
(ii) Roches moutonnees. 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 and 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 structure 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 meltwater 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 axis of each drumlin parallels the direction of ice movement and thus serve 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-Fluvial 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) Kames 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 filled 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) Erratics. 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 ard particularly those lying near about the ice terminus are ordinarily found to contain a number of depressions, some of which may give juse to lakes or swamps. Such hollows are known as kettles.
(vi) Varves, These are layered clays alternating with coarser and finer sediments.
Other Important-Features Associated with Glacier
(i) Nivation. It is the process of quarrying of rocks mostly by frost action.
(ii) Ablation. It includes processes both evaporation and the melting of snow and ice.
(iii) Calving. Within fiords, glaciers come in contact with marine water and blocks of ice are found to break off from the mass of the glacier. This process of wastage of glacier is known as calving.
(iv) Serace. Similar to a waterfall in a river, in a steeper section of the valley, the glacier is broken up into rugged ice-pinnacles and is known as serace.
(v) Iceberg. These are floating ice-hill on the sea-water.
(vi) Crevasses. These are cracks formed due to differential movement within the mass of glacier. In German, they are known as Bergschrund.
(vii) Nunatak. A rock mass which projects through an ice sheet, generally found at the margins of a sheet where the ice is thinnest, is known as nunatak.