Glaciers of the past

Glaciations, periods of cold climate in which the glaciers all over the planet are characterized by a great expansion, and in particular, the formation of large ice-sheets are studied and are well known, with regard to the more recent period, the Quaternary period, but the entire history of the Earth is studied with climatic oscillations, with alternating hot and cold periods, and episodes of glacier advance and retreat. The study of very ancient ice deposits in North America, Africa, Australia enabled the discovery of the traces of the most ancient glaciation, over 2 billion years ago. Very ancient ice deposits were found in Africa and in Australia and can be dated back to 900, 750 and 600 million years ago, as evidence of the same number of glaciations: the extension and duration of these glaciations, however, are not known precisely, as such ancient deposits are preserved only in small discontinuous slivers, and do not enable a reconstruction on a vast scale.
Other glaciations took place in the upper Ordovician period, 450 million years ago, ice deposits and roches moutonnées have been found in the Sahara desert where once there must have been a vast ice-sheet, twice as large as Antarctica at present and, at the time of the Permian-Carboniferous passage, approximately 300 million years ago, with glacial deposits reaching a thickness of 900 m in South Africa, there is evidence of a large ice-sheet that not only covered the Antarctic but also Southern Africa, Madagascar and most of India and Australia (naturally, to study these ancient deposits, it must be borne in mind that the positions of lands above sea level were very different from the present, and so were the positions of the poles). No traces of glaciations have been found during the Mesozoic age , however there is evidence of cold periods during the Cenozoic Era, 65 to 22 million years ago. The Antarctic ice-sheet started to form approximately 15 million years ago, reaching its maximum expansion, which was more extended than at present, 7 to 4.4 million years ago. The Arctic Ice Sheet instead, started forming only 2.6 million years ago, date of the start of the last “ice age”, often called “Quaternary”, but actually starting in the Pliocene period and continues during Quaternary (beginning 1.8 million years ago).

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