Magnetostratigraphy indicates that Early Pleistocene glaciations in North America, instead of forming one continuous ice mass from Atlantic to Pacific as they did in the Late Pleistocene, were characterized by eastern and western ice masses separated by a 2000 km wide north-south ice-free corridor down the centre of the continent. We argue, therefore, that the area covered by ice during periods of glaciation, and hence probably ice volume, in North America was considerably less in the first 2 Ma of the late Cenozoic than it was in the last 0.7 Ma. This is consistent with delta 18O records of ocean cores indicating the ice volumes were much less in the earlier than in the later part of the Cenozoic Ice Age.

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