The Laurentide ice sheet covering northern North America, 20,000 years ago Ice ages are times when the climate cools down so much that ice sheets form at the poles and in mountainous areas and spread out. The most recent ice age began around 2.6 million years ago, at the beginning of the Pleistocene Epoch, in the Quaternary Period. There have been four other times in Earth's history when ice ages occurred. On a number of occasions during the Quaternary glaciation, great ice sheets have spread southwards and covered much of northern Europe, North America and Asia. In between these glacial periods there have been episodes of warmer climate, called interglacials, during which the ice melted and retreated northwards again. We are probably currently living through an interglacial.
The David Glacier, Antarctic ice sheetDuring glacial periods of the Quaternary glaciation, the Arctic ice cap of sea ice extended southwards and the Greenland ice sheet thickened and expanded. Glaciers in mountainous areas spread out to form ice sheets covering the northern half of North America and large parts of Europe and Asia. These ice sheets were up to three kilometres (two miles) thick in places. Some areas in Alaska and Siberia remained ice-free because they were too dry. The last glacial period, commonly (but mistakenly) called the "Ice Age", began about 126,000 years ago, reached its height around 26,500 years ago (the Last Glacial Maximum) and ended only approximately 11,500 years ago.
There are more than 100,000 mountain glaciers in the world. Australia is the only continent where there are none.
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