Glacial Deposits and glacial landform
The regions that were formerly covered by ice are characterized by the following features;
- The topography generally lacks an organized drainage network.
- There are hollows with no outlets some of which form lakes.
- In some places, there are isolated hills, in others long and winding ridges, none of which seem to have much relation to the underlying bedrock.
- In some places, the bedrock is deeply buried, sometimes 50 meters or more, below the glacial drift.
- The ground over the bedrock is mostly a heterogeneous mixture of sand, clay, pebble, and boulders. Many of the pebbles and boulders differ entirely in composition from the bedrock.
Glacial deposits are of two types: (i) those deposited directly by the glacier, are known as “till”, and (ii) materials deposited by glacial melt-water are called “fluvioglacial deposited”. Deposits of till are a mixture of sand, clay, pebbles, and boulders. This material, in general, is heterogeneous and unassorted with not stratification. The pebbles and boulders are commonly striated and they differ markedly in composition from the bedrock. The glacial drift does not exhibit chemical weathering.
Glaciers may transport huge rock boulders, many thousands of tons in weight. When ice melts, they are left behind great distances away from their natural bedrock. Such boulders are called “Erattic Boulders”. These deposits are usually well sorted and stratified accumulation of silt, sand, and gravel.
(1). Moraines: Moraine is a material drifted along the moving glacier, hereinafter left behind the moving glacier. Moraines are the debris and rock material left behind by the moving glacier. Just like a river takes the sand and silt along with itself, and finally, scatter it in deltas. Just like a river the glaciers transport all sorts of debris, dirt, and boulders that eventually remain behind the moving glacier in order to form moraines. Ridges or layers of till are called ”Moraines”, they are of five types: (i) Supraglacial Moraine (includes Lateral and Medial moraines), (ii) ground moraine, (iii) lateral moraines, (iv) medial moraines, and (v) terminal moraines.
- Supraglacial Moraine: A supraglacial moraine is material on the surface of a glacier. Lateral and medial moraines can be supraglacial moraines. Supraglacial moraines are made up of rocks and earth that have fallen on the glacier from the surrounding landscape. Dust and dirt left by wind and rain become part of supraglacial moraines.
- Ground Moraines: A layer of till deposited beneath the moving ice on the ground is called the ”ground moraine”. Ground moraines fill low spots and old streams channels thereby creating a leveling effect.
- Lateral Moraines: These are included in supraglacial moraine. The material that falls from the valley walls, accumulates on the sides of a glacier. When the glacier disappears these materials are left as ridges along the sides of the valley. Such deposits are called ”lateral moraines”.
- Medial Moraines: Medial Moraine is also a supraglacial moraine. When two glaciers meet, a medial moraine is formed by the union of two lateral moraines.
- Terminal Moraines: At the terminus of a glacier where the ice starts melting, the rock debris is deposited in the form of a ridge that extends across the valley. Such deposits are called ”terminal moraines” or ”end moraines”.
- Moraine in Transit:
(2). Outwash Plaines: In front of the end moraines, the streams of meltwater deposit sediment producing stratified deposits of sand, silt, and gravel. Such deposits constitute ”outwash plains”.
(3). Kettle Holes: These are basin-like depressions found in the areas of both till and outwash plains. The diameter of kettle holes ranges from a few meters to a few kilometers. They commonly contain water. These depressions are created when the masses of buried ice melt.
(4). Drumlins: Drumlins are small, smooth, elliptical hills of till that lie parallels to the direction of ice movement. Unlike Roche moutonee the uphill side of the drumlins are steep and the downhill sides are gently sloping. They may be 20-30 meters high and a kilometer long. Drumlins are not found singly but they occur in clusters thereby forming drumlin fields. They are believed to have formed by a subglacial shaping of an accumulated till into streamlined forms.
(5). Eskers: Eskers are long winding ridges of stratified drift found in the middle of ground moraines. They run for kilometers in a direction more or less parallel to the direction in which ice moved. Eskers are formed due to the deposition of gravels and sand by the englacial and subglacial streams. in many areas, they are mined for sand and gravel.
Local melting of ice of glaciers produces streams of water. If these streams flow beneath the glaciers, they are called ”subglacial streams” and if these flow on or within the glaciers, they are called ”englacial streams”.
(6). Kames: Kames are hillocks of stratified drift which are formed at the edges of the retreating ice by glacial streams. These streams fall from a height and deposit sand and gravel along the margin of the glacier as alluvial cones.
(7). Varves: Varves are thinly laminated deposits formed in facial lakes. They consist of alternation of light coloured bands of silt and dark coloured bands of clay. The former gets deposited during the summer season while the later in winter. Thus each pair of varves corresponds to one year of deposition. The thickness of a varve may vary from a very small fraction of a centimeter to 0.75 cm.
(8). Buried Valley: Buried valleys are the ancient deep valleys, which are excavated in the bedrock by glacial erosion and are filled back subsequently with glacial drift. The present-day surface topography gives no clue to their existence. The rivers which are flowing in these areas may have no relation to the buried valleys. Such valleys create unexpected problems for civil engineers.