554 



CENOZOIC ERA-AGE OF MAMMALS. 



curves about the Great Lakes and through Iowa, and thence northwest- 

 ward on the north side of the Missouri, through Dakota, into British 

 America. The discovery of this moraine, which we owe chiefly to 

 Ohamberlin and Upham, must be regarded as a complete demonstra- 

 tion of the existence of the ice-sheet. In the map (Fig. 931) the 



Ice-sheet 

 Boundary. 



B Second Ice- 

 sheet Moraine. 

 [7/| Direction of 

 Striae. 



E Boundary of 

 Lake Agassiz. 

 B Appalachian 

 Range. 



Fig. 931.— Map showing the Extreme Boundary of the Ice-sheet, the Second Ice-sheet Moraine, and 

 the Outlines of Lake Agassiz. 



strong line shows the extreme limit of the first advance, the dotted lines 

 the moraine formed by the second advance of the ice-sheet. In its last 

 retreat many subordinate moraines were left one behind another. We 

 have represented mainly the most advanced. 



Thus we have clear evidence of a second glacial and an interglacial 

 epoch. That this interglacial epoch was of considerable length is 

 shown by the existence of a forest-bed between the two glacial tills 

 (Newberry). Again, since melting and retreat of the ice-sheet must 



