HOW THE GREAT LAKES WERE BUILT. 



167 



all the way from the head of Lake Michigan. This gentle defor- 

 mation of the surface of the country when carried over such long 

 distances gives rise to the great physical reliefs of the mountain 

 regions of the north and east, which were much lower before the 

 lake epoch than now, as is apparent if the tilting be straightened 

 out, as may be seen in the sections given. Furthermore, the char- 

 acter of the river courses at the surface of the country north of 

 the lake regions indicates that even in the ice age the relatively 

 high reliefs north of the lakes did not obtain. 



The Gulf Epoch. In the re- elevation of the lake district 

 after the post-glacial submergence, when the continent was high 

 enough to partially inclose a large gulf, already referred to, to 

 which the name of Warren Water has been given, or more correctly 

 Gulf, there were several water connections through the valleys 

 to the south and west of it. Its last stage as one body of water 

 is marked by the Forest beach, as shown on the map (Fig. 13). 

 Upon the further rise of the land the surface of Warren Gulf fell 

 below the level of the Forest beach for a depth of a hundred 

 and fifty feet. The movement was gradual, without striking 



Fig. 12. Section from the Michigan Highlands back of Alpina to the Laurentian 

 Highlands beyond Lake Nipissing. Length of section, three hundred and thirty miles. 

 The tilted beach represents the deformation of the Algonquin beach. 



interruptions, for there is no intervening strand, as the water did 

 not remain at one level long enough to leave beaches. Warren 

 water was now broken up into two great gulfs ; the one called 

 the Algonquin, occupying the basins of Lakes Superior, Michigan, 

 and Huron, and opening to the northeast, through the strait to 

 the Ottawa Valley, as is shown on the map (Fig. 13) ; the other 

 was the Lundy Gulf, occupying most of Erie basin, and extend- 

 ing over the Ontario Valley at a great height. In the region of 

 Nipissing Strait the two gulfs united. 



From the deserted shores of these waters, which are now tilted 

 up so much to the northeast, free communication between the lake 

 region and Hudson Bay is indicated, for the Laurentian high- 

 lands are now rarely more than fifteen hundred feet above the sea 

 and commonly less. Of course, there was free communication to 

 the Atlantic Ocean by way of the St. Lawrence and also south- 

 ward of the Adirondacks. These various characteristics and 

 changing conditions would require a volume to tell all that we 

 know about them, and this has been partly done in Duration of 



