Glaciers

The snowfields on mountain regions are con­stantly being replenished with fresh falls of snow, the weight of which compresses the underlying material into firn, or neve.

This material is com­posed of ice crystals separated from each other by small air spaces. With increasing depth and pres­sure, the firn gradually changes into much denser glacier ice which moves slowly out from the snowfields down existing valleys.

The glacier becomes a river of moving ice, its surface marked by a series of deep cracks or crevasses. The cracks result from the fact that ice under pressure deforms and moves plastically, whereas the upper layers remain rigid and are therefore under ten­sion and they eventually shear.

Transverse cre­vasses often occur where the slope of the glacier increases; these may be intersected by longitudinal crevasses, creating ice pinnacles, or seracs, between them. A large crevasse, known as a bergschrund, may also form near the head of a glacier in the firn zone where the ice pulls away from the mountain wall.

Glacial Abrasion and Plucking

As a glacier moves it erodes the underlying rocks, mainly by abrasion and by plucking. Abrasion involves rock debris frozen into the sole of the glacier acting on the rocks underneath like coarse sandpaper. Plucking happens when the ice freezes onto rock projections, particularly in well-jointed rocks, and tears the blocks out as it moves.

Considerable evidence exists of glacial erosion having taken place during the Pleistocene Ice Age, when glaciers and ice sheets extended over much of northern Europe and North America. At that time ice moved out of the high mountains and spread over the surrounding lowlands. It modified the shape of the land and left various dis­tinctive landforms that can be seen today long after the ice has receded.

In most glaciated valleys it is possible to find rock surfaces that have been grooved and scratched. These striations were caused by angular rock fragments frozen into the sole of a moving glacier. The marks give some indication of the direction of ice movement.

Where a more resis­tant rock projects out of a valley floor it may have been molded by the passage of ice so that it has a gentle slope on the upstream side (which is planed smooth by the glacier) and a steep ragged slope on the lee side (a result of ice plucking). Seen from a distance these rocks were thought to resemble the sheepskin wigs fashionable in early nineteenth-century Europe, and so were named roches moutonnees.

Comes

An aerial view of a glaciated highland reveals large amphitheater-like hollows arranged around the mountain peaks. These great hollows are called corries (cirques in France, and cwms in Wales) and are the point at which glaciers were first formed during an ice age, or where present-day glaciers start in areas such as the Alps or the Rockies. The Aletsch glacier, for example, begins on the south-eastern slopes of the Jungfrau in Switzerland and is fed by several tributary gla­ciers, each emerging from a corrie.

Frost-shatter­ing of the exposed walls of the corries results in their gradual enlargement; this process is accele­rated by subglacial disintegration of the rock, which occurs when water reaches the rock floor through the bergschrund crevasse at the head of the glacier.

During an ice age most corries were probably filled to overflowing with glacier ice, and their walls and floors were subject to vigorous abrasion. When the ice melted a corrie often became the site of a mountain lake, or tarn, with morainic material forming a dam at the outflow lip.

Corries are bordered by several precipitous knife-edged ridges known as arêtes. These develop when the walls of two adjoining corries meet after glacial erosion has taken place from both sides.

When the arêtes themselves are worn back, the central mass may remain as an isolated peak where the heads of several corries meet. The Matterhorn in the Swiss Alps is a peak that was produced in this way.

Glacial Valleys

When a glacier passes through a pre-existing river valley it actively erodes the valley to a character­istic U-shaped profile.

The original interlocking spurs through which the former river wound are worn back and truncated. In this way the valley is straightened, widened and deepened, and its trib­utary streams are left high above the main trough as hanging valleys. The streams in them often plunge down the valley side as spectacular water­falls, as in the Lauterbrunnen valley between Interlaken and the Jungfrau.

Where several tributary glaciers join the head of a major valley, the increased gouging by the extra ice flow results in the formation of a trough end, or steep step in the U-shaped trough.

The floor of a glaciated valley is often eroded very unevenly and elongated depressions may become the sites of long, narrow, ribbon lakes. Some of the deeper ribbon lakes are dammed by morainic material at their outlets, as in lakes Como and Maggiore in northern Italy.