In chapters 2, 3, and 7 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 19

Box 19.4 (p.294) Drilling Through Ice Sheets for a Record of the Past

Box 19.6 (p.305) Causes of Glacial Ages