Qlaciation

Glaciation refers to the work glaciers do: eroding the surface and transporting and depositing rock debris. As glaciers move they drag rock debris incorporated within the ice across the bedrock, grinding and scratching the surface. They also pluck rock material from the bedrock as water freezes and expands, loosening blocks that then freeze to the bottom of the glacier, which pulls out the blocks as it moves. Some of the rock is ground up, producing very fine particles called rock flour, which turns the meltwater from glaciers a milky white. Glaciers move enormous quantities of rock as they gouge, plow, and carry the rock on their way across the terrain.

Valleys carved by glaciers have a U-shaped profile, which distinguishes them from the V-shaped valleys eroded by rivers. A glaciated valley tends to be linear, the result of the glaciers'

being somewhat inflexible as they wear away and truncate ridges that jut into the valley. Thicker glaciers apply more weight on the valley floor, thus grinding away more rock than their tributaries, which are smaller. Larger main glaciers therefore are deeper than the smaller tributaries. After the glaciers melt away, the tributaries are left as a hanging valley high above the main valley, sometimes with scenic waterfalls.

Cirques are steep-sided, rounded hollows carved into the head of a valley. They usually originate at the upper part of a valley where snow and ice accumulate. Through the processes of weathering and erosion of the valley sides by ice (including ice wedging, plucking, and rock falls), the cirque grows. The large amounts of snow that accumulate are converted to ice, which with the rock that falls on it grinds away at the cirque floor, creating the bowl-shaped depression. Two cirques back to back with a sharp ridge separating them form what is called an arête. When several cirques are distributed around a mountain, the enlarging cirques may intersect, creating a central peak called a horn.

Continental glaciers, like valley glaciers, grind the surface of the bedrock, producing grooved, striated, and polished rock. Ice sheets are often thick enough to cover mountain ranges, and as they move over them the ridges are rounded, smoothed, and molded in the direction of the ice movement.

When ice sheets melt they leave behind the rock fragments they have picked up on their trip. The rounded and unlayered debris that is left behind when the glaciers melt is called till. Because some of the rock that valley glaciers carry fell from the valley walls onto the surface of the glacier, the till is composed of both rounded and angular rock fragments.

Glacial erratics are large rocks that have been left behind by the melting glacier and

Gordon Dam Tasmania
The Saskatchewan Glacier, Alberta Canada, 1954 (USGS/McGimsey)

have been transported from a distant place, so that their composition is unlike that of the bedrock they sit on. Once the source of the erratic has been determined, it then can be used to indicate the direction and movement of the glacier that transported it.

Moraines are deposits of till left behind after the glacier has receded. In valley glaciers, rock debris that has fallen onto the sides of the glacier from the valley wall creates what are called lateral moraines; they are marked as distinctive ridgelike piles of till along the side of the glacier. Where tributary glaciers come together, the adjacent lateral moraines join, forming a long ridge known as a medial moraine. Large glaciers can have several medial moraines built by numerous tributaries that join the main glacier.

When a glacier appears to be stationary because the forward motion and melting are equal, an end moraine is built up in front of the glacier. A terminal moraine is an end moraine built during the farthest advance of a glacier. Recessional moraines are end moraines that are created when a retreating glacier stops temporarily. When glaciers begin to move again, they override and destroy previous moraines, causing signs of the older movements to be lost.

Under some circumstances advancing ice sheets may shape previously deposited till into streamlined, elongate hills called drumlins. The long axis of the drumlin is parallel to the direction of ice movement.

As glaciers melt, large numbers of meltwa-ter streams leave the ice mass, carrying substantial amounts of sediment that had been incorporated within the glaciers. The sediments deposited by these streams are spread over the landscape in front of the glacier, filling in irregularities on the surface and forming an outwash plain. They are easily recognized because they are well sorted and layered, while till is unlayered and unsorted.

An esker is a long, sinuous ridge that forms beneath a large glacier in a tunnel under the ice. After the glacier melts away they are prominent, standing up above the glaciated surface; often roads are built on their surfaces. Eskers can be as high as 30 m, and their sorted, layered, and cross-bedded sediments are mined for sand and gravel.

As the glacier retreats, large blocks of stagnant ice may be covered with sediment; the ice eventually melts, forming depressions called kettles. These kettles fill with water forming small lakes, and they are often numerous, as in the upper Midwest. Glaciers are responsible for a variety of lakes, such as tarns, which form in cirque bowls after the glacier has melted away. Other lakes form in gouged-out depressions on bedrock. As a glacier recedes meltwater can be trapped between the ice front and a moraine, filling up the space between them and forming a lake. This occurs because the weight of the glacier causes the land to slope toward it, forming a lowland that traps the water. Distinctive deposits named varves or rhythmites are formed on the lake bed. In the summer, when melting occurs, coarser sediment is carried into the lake, but in the winter, when melting has diminished and the lake freezes over, only very fine sediment suspended in the water settles out. As a result, a thicker, lighter summer layer and a darker, thinner winter layer of sediment are formed. Each pair represents one year of deposition, and by counting the pairs researchers can determine how long the lake was in existence. By determining the age of the organic material by means of radiocarbon dating and by looking at the entrapped spores and pollen, a good idea can be gained of when and how the climate and vegetation changed.

Some of the sediments found on the out-wash plain are rock flour that settled out of the streams on mudflats and shallow lakes. Later, during dry seasons, the rock flour is easily picked up by the wind, carried away, and deposited as loess. Some of the best agricultural soil in the United States is found on these deposits.

An direct effect of glaciation is the worldwide lowering of sea level by 130 m as water from the oceans became the snow and ice that resided on the land. Rivers flowing across the exposed shelf, some supplemented by glacial water, carved great valleys that are now covered by the risen sea. On the exposed shelf animals lived and died, and today their bones and teeth are dredged up, showing that what is now covered with water was once land. Large areas of the continental shelf were exposed, connecting landmasses and allowing animals to migrate to new places.

The weight of the thick glaciers pushed down the land in a way similar to what happens when you sit on a cushion. When you rise the cushion rebounds, which is similar to what happens to the surface of the earth that had been covered by glacial ice. In the Canadian Arctic, for example, uplift of the land, which is still going on today, is easily seen by the numerous raised beaches that form a flight of stairs up the barren slopes.

Glaciation has had profound effects on the surface of the earth, reshaping it by both erosion and deposition and by interrupting or modifying many normal geologic processes. Glaciers created many thousands of lakes, altered or obliterated old drainage systems while they created new ones, and deposited sediments that became fertile soil in many parts of the world. The effects of glaciation have reached far beyond the margins of the ice and influenced many aspects of the physical and biological world.

—Sidney Horenstein

See also: Climatology; Global Climate Change; Pleistocene Epoch

Bibliography

Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Montgomery, Carla. 1996. Fundamentals of Geology, 3d ed. New York: McGraw Hill Professional Publishing; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill Pub. Co.; Press, Frank, and Raymond Siever. 2000. Understanding Earth. New York: W. H. Freeman and Company; Stanley, Steven. 1999. Earth System History. New York: W. H. Freeman and Company.

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