This report from the United States Geological Survey (USGS) is available online as a pdf file. It describes 'The Great Ice Age', a recent chapter in the Earth's history, when there was a period of recurring widespread glaciations. Mountain glaciers formed on all continents, the icecaps of Antarctica and Greenland were more extensive and thicker than today, and vast glaciers, in places as much as several thousand feet thick, spread across North America and Eurasia.
