Geology Cafe

Glaciers and Glaciation Periods in Earth History

Glaciers are massive streams of ice flowing downslope under the influence of gravity. Glaciers, where they occur, are massive agents of erosion and, when they melt away, can leave uniquely identifiable landscape features. Climate changes (repeating cycles from warm to cold) allow glaciers to grow and advance, or to melt back (retreat). Over the past 4-5 million years there have been many ice ages, with several of them being major episodes lasting tens of thousands of years when continental glaciers covering large portions of North America and Europe (much like Greenland and Antarctica which are still under ice-age conditions). Today we live during an "interglacial period."Prior to about 10,000 years ago, large ice sheets, similar to modern examples on Greenland and Antarctica, covered large portions of North America and Europe. With so much water trapped in continental glaciers, sea level was much lower. Sea level has been steadily rising as the ice sheets melt away. At the peak of the last ice age, sea level was as much as 350 to 400 feet lower that today, and areas that are now continental shelves offshore were exposed as coastal plains. Many mountain ranges, including California's Sierra Nevada, were covered with alpine glaciers.
Alpine glacial features
Click on thumbnail images for a larger view.

Map of California as it probably appeared during the peak of the last ice age of the Pleistocene Epoch, about 18,000 years ago.

Click on images for a larger view.
Satellite view of Sierras Satellite view of the eastern Sierra Nevada Mountains showing snow cover in the high areas. The high Sierras show abundance evidence of intense alpine glaciation during the Pleistocene Epoch.
U-shaped Yosemite Valley. The U shape is strong evidence that an alpine glacier once filled the valley. Glacially polished granite in the high country of Yosemite National Park.
Airliner view of cirques and U-shaped valleys in the high country of the Sierra Nevada. Airliner view of cirques and U-shaped valleys in the high country of the Sierra Nevada.
Bruce F. Molnia, 2004, Glossary of Glacier Terminology: A glossary providing the vocabulary necessary to understand the modern glacier environment: U.S. Geological Survey Open-File Report 2004-1216, http://pubs.usgs.gov/of/2004/1216/
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8/28/2012