The Ice Ages
The change from the later Tertiary to the Pleistocence or major ice ages in the northern hemisphere was not sudden. The average mean temperature, at least that of the temperate regions, began to decline as well as to oscillate. The oscillations increased in frequency and amplitude to culminate ultimately in the major glaciations. Each major glaciation was characterized by several stadials during which the ice front advanced, and several interstadials during which it retreated. Climates comparable to today’s characterized the interglacials, while during the glacial maxima climates colder and more variable than today ’s ranged over the Earth. The foregoing is valid for the northern hemisphere only; in the southern hemisphere the ice ages had begun already in the early Tertiary and there were, as far as can be determined, no stadials or interstadials comparable to those found later in the north (Markov 1969, Dunbar 1970, Margolis and Kennet 1970, Denton et al 1971, Sparks and West 1972). After a temporary upswing during the middle of the Miocene, temperatures continued to decline through the Pliocene into the Pleistocene (Addicott 1969).
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