ANCIENT GLAGJERS. 101 



Michigan, as shown upon our map, but Mr. Frank Leverett 

 has subsequently demonstrated that there is a concentric 

 series of moraines south of this, reaching across the State, 

 (but somewhat obscured by superficial accumulations of 

 loess referred to) and extending nearly to the latitude of 

 St. Louis. 



West of Wisconsin President Chamberlin's " terminal 

 moraine of the second Glacial epoch " bends southward 

 through eastern Minnesota, and, sweeping down through 

 central Iowa, forms, near the middle of the northern part 

 of that State, a loop, having its southern extremity in the 

 vicinity of Des Moines. The western arm of this loop runs 

 through Minnesota in a northwesterly direction nearly 

 parallel with the upper portion of the valley of the Minne- 

 sota, until reaching the latitude of the head- waters of that 

 river, where, in the vicinity of the Sisseton Agency, in 

 Dakota, it turns to the south by an acute angle, and makes 

 a loop in that State, extending to the vicinity of Yankton, 

 and with the valley of the James River as its axis. The 

 western arm of this loop follows pretty closely the line of 

 the eastern edge of the trough of the Missouri River, con- 

 stituting what is called the " Missouri Coteau," which 

 continues on as a well-marked line of hills running in 

 a northwesterly direction far up into the Dominion of 

 Canada. 



One of the most puzzling glacial phenomena in the 

 Mississippi Valley is the driftless area which occupies the 

 southeastern portion of Minnesota, the southwestern part 

 of Wisconsin, and the northwestern corner of Iowa, as de- 

 lineated upon our map. This is an area which, while being 

 surrounded on every side by all the characteristic marks of 

 glaciation, is itself conspicuous for their entire absence. 

 Its rocks preserve no glacial scratches and are covered by 

 no deposits of till, while northern boulders avoided it in 

 their journey to more southern latitudes. 



The reason for all this is not evident in the topography 



