so glaciation is tied to the future of the world's great coastal cities.laciers are responsible for fascinating land forms and for cycles of great change on the earth's surface. the regular cycles of glacial advance and retreat are in a sense the pulse of the earth, or a clock for measuring portions of geologic time. but even more important, glacial cycles contain vital clues, clues to the conditions that future inhabitants of the earth will someday inherit. captioning performed by the national captioning institute, inc. captions copyright 1991, the corporation for community college television annenberg media ♪ for information about this and other annenberg media programs call 1-800-learner and visit us at www.learner.org. annenberg media ♪ when we look at a sunset, we see waves of light energy that have traveled an immense distance to reach our eyes. when we look at an ocean, we see waves of water energy that may have journeyed thousands of kilometers to reach our shores. most waves derive their energy from the wind. as the wind blows over the ocean, some

spectacular mountain ranges such as the alps, the himalayas, and the rockies were sculpted by repeated glaciationyosemite valley, here in the sierra nevada mountains, would have been another nondescript river valley if glaciers hadn't carved it to its present shape. many of the world's most beautiful lakes were gouged out of hard rock by glaciers, including north america's great lakes and the famous lochs of scotland. even the great expanses of rich agricultural soils that blanket china and the soviet union, canada and the united states owe their existence to glaciers. moving glacial ice pulverizes the underlying rock into silt-sized fragments. this silt was eventually transported and concentrated by the wind into the vast fertile soils of today. early scientists didn't really appreciate the important geological role of glaciers. even geologists were convinced that glaciers had never existed

what we call the kalahari desert now was glaciated at that time. that wasn't long ago. that was only about 5% of geologic time. in the southwestern united states, too, there is evidence of a once widespread desert that existed 200 million years ago. fossil dunes are preserved in the upper wall of the grand canyon and in the sandstones of zion national park. the varied surfaces of the shifting sand dunes appear as crisscrossing sets of beds. their large size and coloration from the oxidation of iron show that they formed on dry land. since that time, plant and animal fossils indicate that this region became moist and forested. but in recent geologic time, conditions have become drier in response to plate motions, mountain building, and the development of rain shadows. some lands are deprived of moisture simply becausese they lie a great distance from the ocean, which is the primary source of moisture for rainfall. in western china, the gobi and the takla makan are both locked deep inside a land mass. moist air masses precipitate most of their water before reaching these