A geological phenomena in which massive ice sheets form in the Arctic and Antarctic and advance well toward the equator. Glaciations are characterized by cool, wet climates, thick ice sheets extending well South into what are presently temperate regions. Mountain glaciers in unglaciated areas expand and extend to lower elevations. Sea levels drop due to the presence of large volumes of water above sea level in the icecaps. There is evidence that ocean circulation patterns are disrupted by glaciations
In general, the Earth seems to have been ice free even in high latitudes except during relatively rare glacial periods such as the one from which we are currently emerging.
The causes of glaciations have been much debated since the phenomenon was clearly identified in the 17th century. Modern theories tend to revolve around periodic oscillations in the Earth's orbit; hypothecised periodic changes in solar output; and/or the affects of continental masses drifting into polar regions where Antarctica currently resides.
Known periods of glaciation include the Huronian (2400Ma-2100Ma), Stuartian-Varangian (950Ma-570Ma), Andean-Saharan (450Ma-420Ma), Karoo (360Ma-260Ma), Cenozoic (30Ma-Present). Not every year in each interval was a time of complete or even partial glaciation. The best studied glaciation -- that of the recent past appears to have taken place in at least four separate ice incursions and retreats. Unfortunately the scouring action of each glaciation tends to remove most of the evidence of prior icesheets almost completely except in regions where the later sheet doesn't achieve full coverage. It is probable that glacial periods other than those above have been overlooked because of the paucity of exposed rocks from high latitudes from older time periods. The Varanger glaciation was especially severe and may have extended to the Equator. This has lead to a recent Snowball Earth theory that the Earth froze over completely in the late Proterozoic then thawed very rapidly as trapped water and carbon dioxide were returned to atmosphere.