362 THE ICE AGE IN NORTH AMERICA. 



allel with the first one described, and extended through the 

 Cotean des Prairies of eastern Dakota. A single season's 

 work was sufficient amply to verify Mr. Upham's hypothe- 

 sis, and all subsequent investigation has confirmed it, prov- 

 ing that there was an independent movement of ice down 

 the Minnesota Valley, pushing its lobate front far into the 

 State of Iowa, and some distance beyond the part reached 

 at that time by the general mass on either side. About 

 this lobate margin a vast moraine was built up, whose 

 irregular deposits formed the inclosures containing the lake- 

 lets of that region. Over the intervening area, for a width 

 of seventy miles, there is little left but the ground mo- 

 raine, which is uniform in character, and from which the 

 ice melted so rapidly that there was no chance for the for- 

 mation of lakelets such as characterized the margin where 

 the ice-front remained during the larger part of the 

 period. 



Another class of glacial lakes is due to dams of glacial 

 debris such as were spoken of in a preceding chapter. The 

 lakes of this class are not so numerous as the former, but pre- 

 sent a greater variety of problems for investigation. To 

 appreciate this part of the glacier's work, we must bring 

 again to mind the extent to which erosion had proceeded 

 before the Glacial period began. As already detailed, the 

 vast extent of preglacial erosion is apparent at once upon 

 entering the unglaciated region upon the western fianks of 

 the Alleghany Mountains, where all the rivers occupy nar- 

 row troughs of erosion, hundreds of feet deep, and extending 

 to the very sources of the streams. There are, over that un- 

 glaciated region, few if any waterfalls, simply because the 

 recession of the cataracts which once existed has in most 

 cases proceeded so far that the streams have completed their 

 work, and have already cut their channels through to their 

 extreme limit. The State of Ohio is a portion of the Appa- 

 lachian uplift, and its surface, for the most part, is more than 

 a thousand feet above the sea. The southeastern part of 

 the State is unglaciated, and is characterized by the freedom 



