THE OLD AGE OF CONTINENTS. 131 



of the denudation of the central plateau has occurred 

 during and since the prevalence of glaciers over the 

 northeastern portion of the continent. As to the region 

 east of the Mississippi, however, we know that it was an 

 upland continental area, while even the rocky foundations 

 of the great plateau were accumulating in the bottom of 

 an ocean. It is difficult to conceive how this eastern 

 region, on the advent of the glacier epoch, could have 

 presented a surface greatly less eroded and desert than 

 that which the Colorado valley presents to-day. Vegeta- 

 tion, undoubtedly, held possession of the borders of the 

 water-courses; and it must be remembered the conditions 

 of atmospheric precipitation were, even at that time, as 

 much superior to those of the arid western plains as they 

 now are. Nevertheless, the local sources of humidity 

 had mostly dried up, and the ancient rivers had sunken 

 hundreds of feet into dismal gorges that were destined to 

 be their graves. Traces of these fossil river-channels are 

 frequently encountered. Dr. Newberry has pointed out 

 their existence in Ohio ; General Warren has indicated 

 their presence in Wisconsin, Minnesota and Dakota. The 

 latter has also shown that a depression of the northeast- 

 ern region of the continent, which is even now in prog- 

 ress, has turned northward and eastward the drainage 

 of Winnipeg and other lakes which once poured their 

 surplusage through the Minnesota and Mississippi rivers. 

 The great glacier, in its movement over the surface of 

 the Northern States, together with changes of level and 

 the action of torrents of water springing from the bosom 

 of the dissolving ice-field, has totally transformed the 

 face of this portion of the continent. The ancient river- 

 courses have been filled ; the rugged, eroded and naked 



