Any period of time during which glaciers covered a large part of the earth's surface; SYN. glacial period, glacial epoch.
Any period of glaciation occurring in the Earth's history, but particularly that in the Pleistocene epoch, immediately preceding historic times. On the North American continent, glaciers reached as far south as the Great Lakes, and an ice sheet spread over N Europe, leaving its remains as far south as Switzerland.
There were several glacial advances separated by interglacial stages during which the ice melted and temperatures were higher than today.
Formerly there were thought to have been only three or four glacial advances, but recent research has shown about 20 major incidences. For example, ocean-bed cores record the absence or presence in their various layers of such coldloving small marine animals as radiolaria, which indicate a fall in ocean temperature at regular intervals. 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 change of attitude in relation to the Sun, that is, the way it tilts in a 41,000-year cycle and at the same time wobbles on its axis in a 22,000-year cycle, making the time of its closest approach to the Sun come at different seasons; and (2) the 92,000-year cycle of eccentricity in its orbit round the Sun, changing it from an elliptical to a near circular orbit, the severest
period of an ice age coinciding with the approach to circularity. There is a possibility that the Pleistocene ice age is not yet over. It may reach another maximum in another 60,000 years.
The theory of ice ages was first proposed in the 19th century by, among others, Swiss civil engineer Ignace Venetz 1821 and Swiss naturalist Louis Agassiz 1837. (Before, most geologists had believed that the rocks and sediment they left behind were caused by the biblical flood.) The term “ice age” was first used by botanist Karl Schimper in 1837.