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in geology, a cold episode during an ice age, or glacial period. An ice age (q.v.) is a portion of geologic time during which a much larger part of the Earth's surface was covered by glaciers than at present. The Pleistocene Epoch (1.6 million to 10,000 years ago) is sometimes called the Great Ice Age, or Glacial Age, because during that epoch, ice sheets developed periodically over vast areas of northern and mid-latitude North America and Eurasia. The advance of these ice sheets would be followed some tens of thousands of years later by their melting away and then by another advance. This cyclical process occurred several times during the Pleistocene Epoch, and the successive glacial advances and retreats, which were evidently responses to fluctuations of the Earth's climate, are known respectively as glacial stages and interglacial stages. The interglacial stages represent periods when the Earth's climate seems to have been as warm as it is at present.