The Quaternary Period is distinguished by a series of lengthy ice ages, or glaciations, that affected the high latitudes and many mountain ranges of the world. Ice sheets that formed in the high latitudes of Canada, Scandinavia, and western Russia spread south, covering major parts of the North American and Eurasian continents. The process took thousands of years to complete, and the ice cover lasted tens of thousands of years before it melted back to the polar regions once again. In some Arctic regions, this ice cover was thousands of meters thick. The lowlands of Beringia, however, were not buried by glacial ice in the Pleistocene. This is quite remarkable, because nearly all other high-latitude lands were repeatedly buried by glacial ice during the Pleistocene.
A band of tundra existed just south of the major ice sheets in North America and Eurasia, at least during the last glaciation, and probably also during previous glacial periods of the Pleistocene. The ice sheets themselves helped create the sort of chilled climatic conditions that favor tundra vegetation. Bitterly cold winds flowed off the ice sheets, creating cold regional climates. When the ice sheets reached their maximum extent during the last glaciation, their outer margins were in the midlatitudes (45-50° N), so during such times, Arctic tundra communities existed much farther south than at any other time.
The evidence for cold climates in the midlatitudes during full glacial periods is not limited to ancient flora and fauna, however. The remains of periglacial (near-glacier) features have been found in many mid-latitude locations in Europe and North America. These features developed in permafrost (permanently frozen ground) that formed for many thousands of years during glacial periods. Periglacial features include ice wedges, patterned ground, and pingos. Ice wedges develop in cracks that form in frozen soils. Frost cracking occurs as soil moisture freezes when air temperatures drop. A very rapid drop in air temperature causes a temperature gradient to develop in the soil. Rapid freezing from the top of the soil downward causes cracks to open up at the surface. Once a crack exists, liquid water freezes as it enters the crack, and ice expansion widens and deepens the crack. As the crack expands around the ice, additional water freezes and expands, eventually forming an ice wedge. Large ice wedges (up to several meters in length) tend to form in silty or peaty soils. Frost cracks on level ground tend to form in polygons with diameters of several meters. The polygons make patterned ground, a characteristic feature of much of the Arctic today, and a relict feature of Pleistocene environments down to the midlatitudes. Pingos, or ice-cored hills, form when a body of ice is pushed up from the permafrost, because of poor drainage of saturated soils or from pressure that builds as water drains into a valley from adjacent hillsides. Remains of collapsed pingos can be found in Britain and elsewhere in northwest Europe.
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