A CLOCK OF RECENT GEOLOGICAL TIME 



361 



area that of the Great Lakes region as a whole. To the north 

 of the ^Lakes in Canada is found a most ancient continent which 

 was in existence when all the area to the southward lay below the 

 waters of the ocean. In a period still very many times as long 

 ago as the events we have under discussion, there were laid down 

 off the shore of this oldland a series of unconsolidated deposits 

 which, hardened in the course of time, and elevated, are now repre- 

 sented by the shales, sandstone, and limestone which we find, one 

 above the other, in the Niagara gorge in the order in which they 

 were laid down upon the ocean floor. The formations represented 



FIG. 387. Map* to show the cuestas which have played so important a part in 

 fixing the boundaries of the Lake basins, and also the principal preglaciaj rivers 

 by which they have been trenched (based upon a map by Grabau). 



in the gorge are but a part of the entire series, for other higher mem- 

 bers are represented by rocks about Lake Erie and even farther 

 to the southward. These strata, having been formed upon an out- 

 ward sloping sea floor, had a small initial dip to the southward, 

 and this has been probably increased by subsequent uptilt, including 

 the latest which we have so recently had under discussion. At 

 the present time the beds dip southward by an angle of less than 

 four degrees, or about thirty-five feet in each mile. 



