M j OST of us who have obtained our ideas of the Pleistocene geography of North America from elementary textbooks of geology, and even many who have gone more fully into the subject, have gained an exaggerated impression of the importance of the so-called and centers of radial outflow in the North American sheet. So exaggerated is this impression that the belief is still held that and glaciers accumulated in situi, or nearly so, as two distinct masses that only later became coalescent. A center of radial outflow is an area from which, it is inferred on the geological evidence of striae and related features, glacier flowed outward in all directions. The existence of such a center implies that an divide, either elongate or nearly circular, formerly stood directly above it. The divide may have been localized either by topography -as shown by radial striae around highland areas such as Newfoundland, Gaspe, and New Brunswick-or by the concentration of snowfall on some part of the glacier's upper surface, even though the glacier itself stood on a low land. The and centers are of the latter type. Three facts about these two centers are important. First, the striae that record these former centers must date from a time very late in the Wisconsin glacial age; for any striae recording earlier directions of flow would have been crossed or erased during the latest flow of the glacier ice. Second, the parts of these two centers are known not to have been fixed but to have shifted through distances of three hundred to four hundred miles within a relatively short time. Third, other centers existed more or less contemporaneously in Newfoundland, Gaspe, and New Brunswick, and possibly also in the District of Patricia north of Lake Ontario; further, the vast territory between Hudson Bay and the Rocky Mountains, and also beneath Hudson Bay itself, may well conceal evidence of still other centers. The and centers therefore were features which were not unique and the existence of which has not been proved for any time earlier than a late part of the Wisconsin age. On the other hand, we know that late in Wisconsin time the Keewatin ice and the Labradorian ice formed a continuous coalescent mass. If this had not been the case, the late, high-altitude shore lines of Lake Agassiz could not have been made, for the lake would have lacked a northern shore and would have drained off to Hudson Bay. These facts point toward the concept of a single sheet that at its maximum occupied most of Canada east of the Rocky Mountains and had a geographical center (but not necessarily a center of radial outflow) in the region of Hudson Bay. This concept is essentially the one put forward by G. M. Dawson and modified by T. C. Chamberlin in I895. Dawson called this single glacier the Laurentide sheet-a more appropriate name than those that have been more commonly used since his time.