214 THE ICE AGE IN NORTH AMEBIC A. 



southeastern Dakota, is the extensiveness of the apparently 

 level areas where the till comes to the surface. This impres- 

 sion is heightened, probably, by the absence of forests, and 

 would very likely be the same in portions of Ohio and 

 Indiana were it not for the timber. James River Valley, in 

 Dakota, is depressed in the center about five hundred feet 

 below the edges, but it is, roughly speaking, seventy miles 

 across, and therefore the slope does not strike the eye. So 

 level is the country, that every special line of glacial accu- 

 mulation is a prominent feature in the landscape, and the 

 various halting-places of the ice in its retreat are readily dis- 

 cerned. Evidently, a lobe of ice for a long time filled the 

 James River Valley, running parallel with that which occu- 

 pies the upper Minnesota Valley, and extended southward 

 into Iowa. The edges of these lobes thinned out along the 

 north-and-south line which runs near the east margin of 

 southern Dakota, and favored the accumulation of the mo- 

 rainic hills, to which we have already referred as the Coteau 

 des Prairies. Professor Todd and others speak of this as a 

 series of terminal moraines formed along the sides of the re- 

 entrant angle, between the two lobes, whose apex penetrated 

 to the vicinity of the Sisseton Agency. Perhaps, however, 

 it would facilitate a proper understanding of the subject to 

 speak of the Coteau des Prairies as a medial moraine, like 

 that east of Green Bay, toward which the glacial debris, 

 carried on the deeper portions of the ice of both the Minne- 

 sota River and James River lobes, gravitated in contrary 

 directions. But, whatever the name, certain it is that, start- 

 ing from the Sisseton Agency, different lines of glacial accu- 

 mulations stretch southward at varying angles, the later 

 accumulations forming the more obtuse angle. Coming up 

 the valley of the James from Yankton, one crosses the old- 

 est of these accumulations (the Altamont Moraine) in the 

 neighborhood of the city itself, and the second (or Gary 

 Moraine) in the neighborhood of Mitchell, sixty miles to the 

 north, having run parallel with it, however, for about thirty 

 miles. The third (or Antelope Moraine) is encountered near 



