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Ice ages are times in earth history when earth's climates were appreciably colder than normal and glaciers covered significantly larger areas of the earth compared to today. Ice ages represent conditions associated with the extreme cold periods and global cooling of an otherwise normally fluctuating global climate system. During an ice age, ice sheets, which normally occur only in high latitude areas close to the north and south poles, extend farther into the lower latitudes. Alpine glaciers, which normally exist at higher elevations, extend to much lower elevations during ice ages.

Ice ages occurred as far back as the Precambrian, and occurred sporadically until the present. Major ice ages occurred during the late Precambrian (800–600 million years ago); late Ordovician and early Silurian (460–430 mya); Pennsylvanian ...

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