Geography · Ch 12 — Landforms and their Evolution
Glaciers
Glaciers
The photograph shows a single, sweeping view down the length of a glacial valley. The valley floor is broad and flat, and it is almost entirely filled by a thick, winding tongue of ice and snow that snakes its way down between the mountain slopes. The ice is not a clean, white sheet — it carries a surface of snow and firn, and its margins are likely darkened by rock debris that has fallen from the cliffs above. The valley walls rise steeply on both sides, and the overall impression is of a massive, slow-moving river of ice confined within a U-shaped trough.
What makes this image instructive is what it does not show. There is no visible stream of water, no V-shaped notch, no meandering channel cut into the valley floor. Instead, the ice occupies the entire width of the valley, from wall to wall, and its surface is smooth and unbroken. This is the key visual contrast with a river valley: a river flows in a narrow channel at the bottom of a valley, while a glacier fills the whole trough. The winding course of the ice mass also hints at the fact that glaciers follow the pre-existing drainage lines of the mountains, but they reshape them — widening and deepening them into the characteristic trough form. …
What a Glacier Is
A glacier is a mass of ice that moves over land. The movement can take two broad forms. A vast sheet of ice spreading over the land is called a continental glacier; if such a sheet spreads over the plains at the foot of mountains, it is called a piedmont glacier. Alternatively, ice can flow as linear streams down mountain slopes in broad, trough-like valleys — these are mountain and valley glaciers.
The key point is that glacial movement is slow, nothing like the flow of water. A glacier may move only a few centimetres to a few metres a day, sometimes even less. The basic driving force is simply gravity.
Why Glacial Erosion Is So Powerful
Erosion by glaciers is tremendous because of the friction caused by the sheer weight of the ice. As a glacier moves, it drags along material — usually large, angular blocks and fragments — across the floors and sides of valleys. This causes great damage through two processes: abrasion (scraping and grinding) and plucking (tearing blocks of rock away).
Glaciers can damage even un-weathered rocks. Over long periods they can reduce high mountains into low hills and plains. As the glacier keeps moving, debris is removed, the divides between valleys get lowered, and eventually the slope becomes so gentle that the glacier stops moving altogether. What is left behind is a mass of low hills, vast outwash plains, and other depositional features.
In India, many glaciers move down the slopes and valleys of the Himalayas. They can be seen in the higher reaches of Uttarakhand, Himachal Pradesh, and Jammu and Kashmir. The river Bhagirathi is fed by meltwaters from under the snout (called Gaumukh) of the Gangotri glacier, and the Alkapuri glacier feeds the Alakananda river. The Alakananda and Bhagirathi join near Devprayag to form the Ganga.
Erosional Landforms
Cirques
Cirques are the most common landforms in glaciated mountains, often found at the heads of glacial valleys. They are formed when accumulated ice cuts into the mountain while moving down. A cirque is a deep, long, and wide trough or basin with very steep, concave to vertically dropping high walls at its head and sides. After the glacier disappears, a lake often remains within the cirque — such lakes are called cirque lakes or tarn lakes. Sometimes two or more cirques occur one leading into another down below, in a stepped sequence.
Horns and Serrated Ridges
Horns form through the headward erosion of cirque walls. When three or more radiating glaciers cut headward until their cirques meet, high, sharp-pointed, steep-sided peaks called horns are produced. The Matterhorn, the highest peak in the Alps, and Everest, the highest peak in the Himalayas, are both horns formed by the headward erosion of radiating cirques.
The divides between cirque side walls or head walls get narrowed by progressive erosion and turn into serrated or saw-toothed ridges, sometimes called arêtes, with very sharp crests and a zig-zag outline.
Glacial Valleys / Troughs
Glaciated valleys are trough-like and U-shaped, with broad floors and relatively smooth, steep sides. They may contain littered debris, or debris shaped as moraines giving a swampy appearance. There may be lakes gouged out of the rocky floor, or lakes formed by debris within the valleys.
Hanging valleys may occur at an elevation on one or both sides of the main glacial valley. The faces of divides or spurs of such hanging valleys, where they open into the main valley, are often truncated to look like triangular facets. Very deep glacial troughs filled with sea water, forming shorelines in high latitudes, are called fjords or fiords.
A useful way to think about the difference between glacial and river valleys: glacial valleys are U-shaped with broad, flat floors and steep sides, while river valleys are typically V-shaped with narrower floors.
Depositional Landforms
Glacial Till and Outwash Deposits
The unassorted coarse and fine debris dropped by melting glaciers is called glacial till. Most rock fragments in till are angular to sub-angular in form.
Streams form from melting ice at the bottom, sides, or lower ends of glaciers. Some rock debris small enough to be carried by these meltwater streams is washed down and deposited. Such deposits are called glacio-fluvial deposits or outwash deposits. Unlike till, outwash deposits are roughly stratified and assorted, and the rock fragments are somewhat rounded at their edges.
Moraines
Moraines are long ridges of glacial till deposits. There are several types:
- Terminal moraines — long ridges of debris deposited at the end (toe) of the glacier.
- Lateral moraines — formed along the sides, parallel to the glacial valley. They may join a terminal moraine to form a horse-shoe shaped ridge. There can be many lateral moraines on either side of a glacial valley. They partly or fully owe their origin to glacio-fluvial waters pushing materials to the sides of glaciers.
- Ground moraines — when valley glaciers retreat rapidly, they leave an irregular sheet of till over their valley floors. Such deposits, varying greatly in thickness and surface topography, are called ground moraines.
- Medial moraines — the moraine in the centre of the glacial valley, flanked by lateral moraines. They are imperfectly formed compared to lateral moraines, and are sometimes indistinguishable from ground moraines.
Eskers
When glaciers melt in summer, water flows on the surface of the ice, seeps down along the margins, or moves through holes in the ice. This water accumulates beneath the glacier and flows like streams in a channel beneath the ice. Such streams flow over the ground — not in a valley cut into the ground — with ice forming their banks. Very coarse materials like boulders and blocks, along with some minor fractions of rock debris, settle in this valley of ice beneath the glacier. After the ice melts, these deposits remain as a sinuous ridge called an esker.
Outwash Plains
The plains at the foot of glacial mountains, or beyond the limits of continental ice sheets, are covered with glacio-fluvial deposits in the form of broad, flat alluvial fans. These fans may join to form outwash plains of gravel, silt, sand, and clay. …
Masses of ice moving as sheets over the land -- a continental glacier, or a piedmont glacier if a vast sheet of ice is spread over the plains at the foot of mountains -- or as linear flows down the slopes of mountains in broad trough-like valleys (mountain and valley glaciers) are called glaciers. Glacier movement is slow, unlike water flow -- from a few centimetres to a few metres a day or even less or more -- and glaciers move basically because of the force of gravity. Erosion by glaciers is tremendous because of the friction caused by the sheer weight of the ice. Material plucked from the land by glaciers (usually large-sized angular blocks and fragments) gets dragged along the floors or sides of the valleys, causing great damage through abrasion and plucking; glaciers can cause significant damage even to un-weathered rocks and can reduce high mountains into low hills and plains. As glaciers continue to move, debris gets removed, divides get lowered, and e …