Paleoclimatologists that study ancient climates of the earth have identified three distinct periods of glaciation: 1.) a period of glaciation around 600 million years ago about which little is known, 2.) glaciation that effected areas of Africa, India and Australia approximately 275 million years ago, and 3.) the most recent period, which began about 1.5 million years ago and ended (or at least subsided) within the last 15,000 years.
http://www.uvm.edu/whale//GlaciersHowFrequent.html
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