Glacial epoch

(geology) Any of the geologic epochs characterized by an ice age; thus, the Pleistocene epoch may be termed a glacial epoch. Generally, an interval of geologic time which was marked by a major equatorward advance of ice; the term has been applied to an entire ice age or (rarely) to the individual glacial stages which make up an ice age.

An informal reference to a time during the history of the Earth when there were larger ice sheets (continental size) and mountain glaciers than today. The most recent glacial epoch, better known as the Pleistocene glacial epoch, and also by the older term Quaternary period, encompassed at least the last 3,000,000 years.

Many side effects resulted from the existence of these ice sheets and glaciers, including climate changes, sea-level rise and fall, depressions of the Earth's crust, and large-scale migrations of plants, animals, and humans as well as mass extinctions. Mountain landscapes were sculptured by glaciers, and erosional and depositional landforms were formed. Ocean temperatures were cold during glaciations and warm at times of interglacials. Early human evolution, development, and migrations resulted from the ever-changing climates closely related to glacier advances and retreats.

Glacial epochs seem to recur at intervals of 200,000,000 to 250,000,000 years. In overall occurrence, all the glacial epochs that have ever occurred occupy only 5 to 10% of all geologic time. During major glacial epochs, great ice sheets formed in the high latitudes and spread out to cover as much as 40% of the Earth's, land surface. Accompanying drops in temperature during some glacial epochs may have been as much as 25°F (14°C) in the mid-latitudes. During a glacial epoch, major glaciations are short-lived, each lasting less than 10,000 years, with the interglacials persisting for only about 10,000 years, so that for most of an epoch, the ice sheets either grow or diminish in size. The Pleistocene glacial epoch was distinguished by seven or eight glacial advances within the last 700,000 years. Its last glaciation ended about 9000 years ago in Fennoscandia, and less than 8000 years ago in north-central Canada. See also Glacial geology; Glaciology; Pleistocene.