Periods when ice has accumulated at the poles and the continents have been glaciated repeatedly. Exactly why glaciation occurred is not clear. There are suggestions of a middle Precambrian glaciation about 2300 Ma in N. America, S. Africa, and Australia. More information exists to suggest that the Earth was glaciated between 950 and 615 Ma ago, and there are at least two glacial horizons in Africa, Australia, and Europe. There is good evidence for a glaciation at the end of the Ordovician in N. Africa, but glacial deposits described from elsewhere at this period are problematical, so the extent of the glaciation is not known. The Permo–Carboniferous glaciation of S. America, S. Africa, India, and Australia was widespread and is well documented. There is no evidence for further glaciation until the Quaternary. Suggestions have been made for other ice ages during the Palaeozoic but evidence for them is sparse. The Pleistocene ice age is the best documented, but there is undoubted evidence of earlier glaciations in the geologic record.