ANCIENT GLACIERS. 115 



from a pretty general southward flow of a semi-fluid from 

 the two subcentres mentioned, meeting with the obstruc- 

 tions of the Adirondacks in northern New York and of 

 the broader Appalachian uplift in northern Pennsylvania. 



How far south the area of glacial accumulation may 

 have extended cannot be definitely ascertained, but doubt- 

 less at an early period of the great Ice age the northern 

 portions of the Appalachian range in New York, New 

 EDglancl, New Brunswick, and Nova Scotia became them- 

 selves centres of dispersion, while only at the height of 

 the period did all their glaciers become confluent, so that 

 there was one continuous ice-sheet. 



In the western portion of the area covered by the Lau- 

 rentide Glacier, the depression occupied by the Great 

 Lakes, especially Lakes Michigan and Superior, evidently 

 had a marked influence in directing the flow of ice during 

 the stages which were midway between the culmination 

 of the Ice period and both its beginning and its end. 

 This would follow from the great depth of these lakes, 

 the bottom of Lake Michigan being 286 feet below sea- 

 level, and that of Lake Superior 375 feet, making a total 

 depth of water of about 900 and 1,000 feet respectively. 

 Into these oblong depressions the ice would naturally 

 gravitate until they were filled, and they would become 

 the natural channels of subsequent movement in the di- 

 rection of their longest diameters, while the great thick- 

 ness of ice in them would make them the conservative 

 centres of glacial accumulation and action after the ice 

 had begun to retreat. 



These deductions from the known nature of ice and 

 the known topography of the region are amply sustained 

 by a study of the detailed map showing the glacial geol- 

 ogy in the United States. But on this we can represent 

 indeed only the marks left by the ice at various stages of 

 its retreat, since, as already remarked, the marks of each 

 stage of earlier advance would be obliterated by later for- 



