Glaciers

In a cold climate with abundant snowfall, the snow of winter may not completely melt or evaporate during the following summer, and so a deposit of snow accumulates from year to year. Partial melting and continual increase in pressure cause the lower part of a snow deposit to change gradually into ice. If the ice is sufficiently thick, gravity forces it to move slowly downhill. A moving mass of ice formed in this manner is called a glacier. Approximately 10 percent of the earth’s land area is covered by glacial ice at the present time.

Today’s glaciers are of two principal types:

Valley glaciers – found, for instance, in the Alps, on the Alaskan coast, in the western United States – are patches and tongues of dirty ice lying in mountain valleys. These glaciers move slowly down their valleys, melting copiously at their lower ends; the combination of downward movement and melting keeps their ends in approximately the same position from year to year. Movement in the faster valley glaciers (a few feet per day) is sufficient to keep their lower ends well below timberline.

Glaciers of another type cover most of Greenland and Antarctica: huge masses of ice thousands of feet thick and thousands of square miles in area, engulfing hills as well as valleys, and appropriately called continental glaciers or ice caps. These, too, move downhill, but the “hill” is the slope of their upper surfaces. An ice cap has the shape of a broad dome, its surface sloping outward from a thick central portion of greatest snow accumulation: its motion is radially outward in all directions from its center. The icebergs of the polar seas are fragments that have broken off the edges of ice caps. Similar sheets of ice extended across Canada and northern Eurasia in relatively recent geological history.

Apparently a glacier moves by internal fracture and healing in the crystals of solid ice as well as by sliding along its bed. Like a stream, a glacier carries along rock fragments which serve as tools in cutting its bed. Some fragments are the debris of weathering that drop on the glacier from its sides; others are torn from its bed when melted water freezes in rock cervices. Fragments at the bottom surface of the glacier, held firmly in the grip of the ice and dragged slowly along its bed, gouge and polish the bedrock and are themselves flattened and scratched. Smoothed and striated rock surfaces and deposits of debris containing boulders with flattened sides are common near the ends of valley glaciers. Where such evidence of the grinding and polishing of ice erosion is found far from present-day glaciers, we have reason to infer that glaciation was present there in the past.

Valley glaciers form in valleys carved originally by streams. A mountain stream cuts like a knife vertically downward, letting slope wash, slumping, and minor tributaries shape its valley walls; by contrast, a glacier is a blunt erosional instrument which grinds down simultaneously all parts of its valley floor and far up the sides as well. Effects of this erosion are best seen in valleys that have been glaciated in the past but in which glaciers have dwindled greatly or disappeared. Typically such valleys have U-shaped cross sections with very steep sides, instead of the V shapes produced by stream erosion. Their heads are round, steep-walled amphitheaters called cirques, in contrast to the small gullies at the heads of stream valleys. Tributary streams often drop into a formerly glaciated valley over high cliffs because a large glacier carves out its channel much more actively than a small one does. A tributary valley left stranded high above its main valley is called a hanging valley and is often the scene of a spectacular waterfall.

Divides between cirques and between adjacent U-shaped valleys tend to be sharp ridges because of the steepness of the valley walls. In general, since valley glaciers produce deep gorges, steep slopes, and knifelike ridges, their effect is to make mountain topography extremely rugged. The earth’s most spectacular mountain scenery is in regions (the Alps, the Rockies, the Himalayas) where valley glaciers were large and numerous several thousand years ago.

The influence of ice caps on landscapes is very different from that of valley glaciers. We cannot, of course, observe directly the effect of existing ice caps on the buried landscapes of Greenland and Antarctica, but larger ice caps that once covered much of Northern Europe and North America have left clear records of their erosional activity, which we can easily see from the rounded hills and valleys, the abundant lakes and swamps so characteristic of these regions. Like a gigantic piece of sandpaper, an ice cap rounds off sharp corners, wears down hills, and fills depressions with debris, leaving innumerable shallow basins which form lakes when the ice recedes.

Glacial erosion is locally very impressive, particularly in high mountains. The amount of debris and the size of the boulders that a glacier can carry are often startling. But in general, on a worldwide basis, the erosional work accomplished by glaciers is small. Only rarely have they eroded rock surfaces deeply, and the amount of material transported long distances is insignificant compared with that carried by streams. Most glaciers of today are but feeble descendants of mighty ancestors, but even these ancestors succeeded only in modifying landscapes already shaped by running water.