172 THE ICE AGE IN A'UR'IH AMERICA. 



northwest course is coincident with the bluffs on the north- 

 east side of the river through Jackson, Kandolph, and Mon- 

 roe counties, 111. 



So far I have traced the bouthern boundary myself, and 

 the information here given is nearly all at first hand. Beyond 

 the Mississippi competent members of the United States 

 Geological Survey have traced the course approximately to 

 the Pacific Ocean. From these data we know that across 

 the State of Missouri the Missouri liiver approximates closely 

 to the glacial limit. The line enters Kansas a little south of 

 Kansas City, and runs nearly west for a hundred miles to 

 the vicinity of Topeka, where it curves northward, crossing 

 the State of Nebraska about one hundred miles west of the 

 Missouri River, and reaching the southern line of Dakota, 

 near the junction of the Niobrara and the Missouri. In 

 eastern Kansas and Nebraska the exact limits of the glaciated 

 area appear, from the reports, to be somewhat difficult of 

 determination. It would seem that the action of water and 

 floating ice was predominant in determining the character 

 of the glacial deposits over that region, and the theory is 

 plausibly suggested by Professor Todd that the extension 

 of the ice beyond the Missouri formed glacial dams across 

 the valleys of the Kansas and Platte Rivers, so as to 

 maintain for a short period temporary lakes of a consid- 

 erable extent, which received and distributed the bowlder- 

 laden fragments of ice, as well as the finer elements of the 

 glacial deposits. The most of the glaciated portion of these 

 States is deeply covered with fine loam, or loess, which is 

 probably a water deposit, and, as we shall hereafter see, is on 

 good grounds believed by Chamberlin and Salisbury to be 

 an " assorted variety of glacial silt directly derived from gla- 

 cial waters." * 



Through Dakota the glaciated region is bounded by a line 

 which runs northward from near the junction of the Niobrara 



* "Preliminary Paper on the Driftless Area of the Upper Mississippi Val- 

 ley," by Thomas C. Chamberlin and Rollin D. Salisbury, in the " Sixth Annual 

 Report of the United States Geological Survey," p. 304. 



