GEOLOGICAL HISTORY OF THE LA URENTIAN BASIN. 399 



result of subaerial erosion it will follow, unless it is found that 

 the deeper portions are the result of glacial action, that the land 

 at the time the streams did their work, must have stood higher 

 than at present, for the reason that the bottom of the depression 

 is now below sea level. Some idea of the smallest amount of 

 elevation necessitated by this hypothesis might be obtained by 

 estimating the gradients of the ancient streams and the amount 

 of elevation required to bring the bottom of the depression up 

 to sea level. 



A study of the hard -rock topography in the valleys of the 

 Ottawa and St. Lawrence and of the present submerged Atlantic 

 border of the continent would also be instructive in this con- 

 nection. The strict correlation of the topographic history of the 

 interior and of the continent's margin may be difificult, but as 

 the two regions are directly connected, valuable results should 

 follow their comparative study. 



The hypothesis that the Laurentian basin is due largely to pre- 

 glacial erosion, necessitates also that the ancient system of 

 river valleys should have been closed in some way so as to form 

 the basins of the present and of former lakes. The closing has 

 been referred to several agencies. An unequal subsidence fol- 

 lowing the period of stream erosion has been postulated. 

 During the Glacial epoch the entire region was ice -covered and 

 only glacial streams of one kind or another could have existed. 

 On the retreat of the ice, when portions of the basin were aban- 

 doned, the drainage is supposed to have been obstructed by the 

 ice itself, as will be noticed below. When the glaciers melted, a 

 vast sheet of debris was left which in many instances filled or 

 obstructed previous drainage lines. Old channels, now deeply 

 buried, have been reported to connect the basins of the various 

 existing lakes, as has already been mentioned, but no similar 

 channel which could have afforded an escape for the waters of 

 the entire basin has been discovered. Here again an acquaintance 

 with the hard -rock topography should give assistance and indi- 

 cate either that such a channel existed or that orographic move- 

 ments have taken place which have obstructed the former drain- 



