Any period of extensive glaciation (in which icesheets and icecaps expand over the Earth) occurring in the Earth's history, but particularly that in the Pleistocene epoch (last 2 million years), immediately preceding historic times. On the North American continent, glaciers reached as far south as the Great Lakes, and an icesheet spread over northern Europe, leaving its remains as far south as Switzerland. In Britain ice reached as far south as a line from Bristol to Banbury to Exeter. There were several glacial advances separated by interglacial (warm) stages, during which the ice melted and temperatures were higher than today. We are currently in an interglacial phase of an ice age. Other ice ages have occurred throughout geological time: there were four in the Precambrian era, one in the Ordovician, and one at the end of the Carboniferous and beginning of the Permian. The occurrence of an ice age is governed by a combination of factors (the Milankovitch hypothesis ): (1) the Earth's…