Ice age

Ice age, any of several periods in geologic time when thick ice caps and glaciers covered large areas of the earth that now have temperate or warm climates. In 1837 Louis Agassiz, a Swiss-U.S. naturalist, was one of the first to argue that a great ice age in the past explained land features associated with glaciation hundreds of miles from permanent ice. Geologists later established that there have been several ice ages. Ice ages occurred in Australia, Canada, China, India, South Africa, and other places more than 700 million years ago, but little is known about them. Another great ice age affected the Southern Hemisphere during the early Permian period, which began c.275 million years ago. The ice ages about which most is known occurred in the Northern Hemisphere during the Pleistocene epoch in the past million years. There were 4 main glacial ages when the ice advanced: the Nebraskan (which began c.600,000 years ago), the Kansan (which began 476,000 years ago), the Illinoian (which began 230,000 years ago), and the Wisconsin (which began 115,000 years ago and ended c. 10,000 years ago). The European names for these glacial ages are the Günz, Mindel, Riss, and Würm. These were separated by 3 interglacial ages during which the weather became warmer and the ice retreated. In North America the interglacials are named the Aftonian, the Yarmouth, and the Sangamon. Geologists do not know whether the close of the Wisconsin glacial age marked the end of the recent ice ages, or whether the present time is the fourth interglacial, preceding a fifth glacial age.