Chapter Overview

In the preceding three chapters you have seen how the surface of the land is shaped by mass wasting, running water, and, to some extent, groundwater. Running water is regarded as the erosional agent most responsible for shaping Earth's land surface. Where glaciers exist, however, they are far more effective agents of erosion, transportation, and deposition. Geologic features characteristic of glaciation are distinctly different from the features formed by running water. Once recognized, they lead one to appreciate the great extent of glaciation during the recent geologic past (that age popularly known as the Ice Age).

Immense and extensive glaciers, covering as much as a third of Earth's land surface, had a profound effect on the landscape and on our present civilization. Moreover, worldwide climatic changes during the glacial ages distinctively altered landscapes in areas far from the glacial boundaries. For instance, water stored as ice in glaciers came from the oceans, so sea levels were lowered and more land was above sea level.

These episodes of glaciation took place within only the last couple million years, ending about 10,000 years ago. Preserved in the rock record, however, is evidence of extensive older glaciations. The chapter on plate tectonics shows how the record of these ancient glaciations supports the theory of plate tectonics.

To understand how glacial erosion and deposition could have created the features regarded as evidence for past glaciations, you must first appreciate how present-day glaciers erode, transport, and deposit material. In other words, you must apply the principle of uniformitarianism to your study of glaciation.

Expanded Readings From Chapter 12

Box 12.4 (p.294) Drilling Through Ice Sheets for a Record of the Past

Box 12.6 (p.305) Causes of Glacial Ages

Learning Objectives

1. Glaciers are bodies of ice that form from the accumulation of snow and advance because of gravity. Snow accumulating on the ground (firn) is a sedimentary rock that coverts to glacial ice, a metamorphic rock, by compaction (pressure) and recrystallization (heat).

2. Glaciers exist as either valley glaciers, occupying old stream valleys at high elevations, or continental glaciers, ice sheets covering wide continental areas at lower elevations.

3. The zone of accumulation lies above the equilibrium line and has perennial snow cover, while ice is lost in the zone of ablation, below the equilibrium line, by evaporation, melting or calving at the terminus. Advancing glaciers have a positive ice budget (accumulation > ablation), while retreating glaciers have a negative ice budget (accumulation < ablation).

4. Valley glaciers move by basal sliding against the bedrock in response to gravity. The lower portion of glaciers exhibits plastic flow from basal sliding and deformation, while the upper portion of a glacier is rigid, exhibiting crevasses because it is brittle. Ice sheets, such as those of Antarctica, exhibit plastic flow almost exclusively and no basal sliding.

5. Rock fragments transported at the base of a glacier are effective erosional agents. Erosion by valley glaciers produces most of the scenery in mountainous regions. Striated and groove bedrock is a common erosional feature of both alpine and continental glaciation. Valley glaciers produce U-shaped valleys, truncated spurs, hanging valleys and rock-basin lakes as they modify old stream valleys. Cirques, horns and arêtes are formed by valley glaciers in combination with weathering and erosion.

6. Glacial deposits are either unsorted, unlayered, angular rock debris called till, or sorted, layered material called outwash that is deposited by meltwater. Moraines are ridges of till formed by valley and continental glaciers while the terminus is stationary. Both can form end, terminal, recessional, and ground moraines. Valley glaciers also form lateral and medial moraines because of rockfalls along their edges and convergence into trunk glaciers. Continental glaciers deposit drumlins, spoon-shaped hills of till with a gentle dip in the direction of ice movement. Outwash forms in front of a glacier from meltwater. Eskers are deposited by continental glaciers from meltwater flowing under the ice.

7. Other depositional features associated with continental glaciers are kettles, rock flour and loess, varved lake deposits, and ice-dammed lakes.

8. Not all glaciation is associated with the Pleistocene. The oldest glaciations occurred about 2.3 billion years ago. Late Paleozoic glaciations are evidence for continental drift, and even the most recent ice ages began 2 or 3 million years ago. Mean temperature was only 5 degrees C lower than current temperatures at the height of Pleistocene glaciations.

9. Much of the current landscape is the result of glacial effects. Scoured bedrock in Canada and thick tills in the central United States were directly caused by continental glaciation. The Great Lakes were eroded by continental glaciers as well. Indirect effects of glaciation are pluvial lakes, such as the Great Salt Lake, formed by abundant rainfall associated with the Ice Ages. As glaciers advanced during the Ice Ages, sea level fell, causing fiords along the coastlines of Alaska, Norway and elsewhere. Uplifted and tilted shorelines around the Great Lakes indicate crustal rebound from removal of the weight of continental ice sheets.

Related Readings

Bennett, M. R., and N. F. Glasser. 1996. Glacial Geology: Ice sheets and Landforms. New York: J. Wiley.

Easterbrook, D. J. 1993. Surface Processes and Landforms. New York: McMillan Publishing Company.

Ehlers, J. 1996. Quaternary and Glacial Geology. New York: J. Wiley.

Hambrey, M. J. 1995. Glacial Environments. Vancouver, B.C.: UBC Press.

Post, A., and E. R. LaChappelle. 1971. Glacier Ice. Seattle: University of Washington Press. (This is a book of photographs.)

Ritter, D. F., R. C. Kochel, and J. R. Miller. 1995. Process Geomorphology. 3d ed. Dubuque, IA: Wm. C. Brown Publishers.

Sharp, R. P. 1989. Living Ice: Understanding Glaciers and Glaciation. New York: Cambridge University Press.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 12:

Boxed Readings