406 THE JOURNAL OF GEOLOGY. 



Former outlets should bear a definite relation to neighboring- 

 shore lines and to sedimentary deposits. The channels leading 

 from former points of discharge merit examination, as here again 

 changes of level may perhaps be detected in the gradients of 

 stream terraces. 



Most of the ancient outlets thus far recognized lead south- 

 ward, but as previously mentioned, a former channel of discharge 

 north of Lake Superior has recently been reported. If this 

 observation is confirmed, it will have an important bearing on 

 questions relating to changes of level and to the position of the 

 ice front during the later stages in the retreat of the glaciers. 



9. Probable effects of a retreatitig ice sheet on drainage. The 

 generally accepted conclusion that glaciers advanced southward 

 and occupied the Laurentian basin during the Glacial epoch and 

 retreated northward toward the close of that epoch, is sustained 

 by a vast body of evidence. As the ice sheet withdrew it left a 

 superficial deposit frequently one or two hundred feet thick over 

 nearly all of the region it abandoned, and pre-glacial drainage 

 lines were obstructed and mostly obliterated. As long as the 

 slope in front of the ice was southward, the drainage from it 

 found ready means of escape, but when the slope was northward 

 towards the ice front, the drainage was obstructed and lakes were 

 formed. 



We have good reasons for believing that the topography of 

 the Laurentian region was essentially the same at the close of 

 the Glacial epoch as it is now, but the broader question of con- 

 tinental elevation is less definite. The inequalities of the surface 

 being essentially as we now find them, it would follow that the 

 first lake formed when the ice retreated to the north of the 

 divide running through central Ohio and central New York, 

 would be small and dependent on minor features in the relief of 

 the land, and would discharge southward. As the ice retreated, 

 the lakes would expand and become united one with another and 

 the larger lakes thus formed would still find outlet across the 

 southern rim of the basin. As the glaciers continued to retreat 

 lower and lower, passes would become free of ice and the lakes 



