WHAT IS THE GLACIATION?

Process by which land is covered by continental and Alpine glacier ice sheets or the period of time during which such covering occurred; several glaciations are required to make up an ice age (such as the Pleistocene). The land is subject to erosion and deposition by this process, which occurred repeatedly during the Quaternary; the process modifies landscapes and affects the level of ocean basins. These periods of colder weather are also called glacials, and the warmer periods between them interglacials. At the onset of colder weather, water is taken up into the ice sheets and glaciers, causing a drop in sea level. Landscapes once covered by ice can be recognized by the smooth rock surfaces and the U-shaped valleys formed by the ice sheets and glaciers and the rock rubble carried along in them. As the climate warmed, the glaciers retreated, the ice melted, and the sea level rose. The ice also deposited various forms of boulder clays, and banks of debris at the sides and ends of glaciers, known as moraines. Beyond the limits of glaciers and ice sheets, extensive layers of outwash sands and gravels were deposited; where these deposits occur in lakes they are called varves. The periglacial zone around the margin of an ice sheet has permanently frozen subsoil, and is occupied by cold-loving plants and animals. Erosion was mainly brought about by solifluxion. The low temperatures and the constant freezing and thawing also affect the soil; these frost effects are called cryoturbation. Particularly characteristic are ice wedges, polygonal cracks in the ground frequently recognizable in air photographs. They are caused by the shrinking of the ground at low temperatures and the filling of the cracks with water, which subsequently expand on freezing to open the crack still further. The last 2 million years have been marked by a series of such glaciations. Broad correlations between the glaciation schemes in different parts of Europe and North America exist. Four ice ages have been figured; in Europe, the first glaciation was at a climax 550,000 years ago. This gradually gave way to the first interglacial (Gunz-Mindel) period lasting about 60,000 years in which warm conditions again prevailed. The second glaciation came along with its climax 450,000 years ago, and the second interglacial (Mindel-Riss) period followed, lasting 200,000 years. The third glacial (Riss) period climax 185,000 years ago was relieved by 60,000 years of interglacial warmth. The fourth (Wurm) and last ice age was at its height 72,000 years ago. The term has also commonly been used to describe the periods of generally cold climate that occurred at intervals during the Quaternary period. It is, however, now clear that ice sheets grew only during parts of these socalled “glacials” (e.g., the Devensian). For this reason, the term “cold stage” is preferable.