GLACIAL STAGE

in geology, a cold episode during an ice age, or glacial period. An ice age (q.v.) is a portion of geologic time during which a much larger part of the Earth's surface was covered by glaciers than at present. The Pleistocene Epoch (1.6 million to 10,000 years ago) is sometimes called the Great Ice Age, or Glacial Age, because during that epoch, ice sheets developed periodically over vast areas of northern and mid-latitude North America and Eurasia. The advance of these ice sheets would be followed some tens of thousands of years later by their melting away and then by another advance. This cyclical process occurred several times during the Pleistocene Epoch, and the successive glacial advances and retreats, which were evidently responses to fluctuations of the Earth's climate, are known respectively as glacial stages and interglacial stages. The interglacial stages represent periods when the Earth's climate seems to have been as warm as it is at present. The succession of glacial stages can be recognized in sequences of geologic strata containing till (i.e., rock, sand, and clay particles deposited by glaciers), with each till layer representing a stage, or episode, when the ice sheets were much more extensive than they are today. Some of the tills are separated from over- and underlying ones by ancient soils or other sediments of nonglacial character. The intervals corresponding to these features are interglacials. Pleistocene strata have been much studied in North America and Europe. Each successive glaciation represented in these strata has been identified and named, though the same glacial episode usually bears a different name in North America than in Europe. Indeed, three regions of Europe have been differentiated, and all have different names for what is essentially the same worldwide sequence of glacial and interglacial stages. The main stages in North America and Alpine Europe from the Pleistocene on are given in the Table. Most major glaciations were themselves composed of smaller pulses of glacial advance alternating with similar periods of retreat. These subcycles, called substages, are known with certainty only for the world's last glacial stage, i.e., the Wisconsin in North America and the Wrm in Alpine Europe. The last of this stage's three or four substages, called the Mankato in North America and Wrm IV in Europe, ended about 10,000 years ago, by which time the world's glaciers had retreated to their present-day dimensions. See also Pleistocene Epoch.