466 J. E. TODD PRE-WISCONSIX CHAN^'ELS IJS DAKOTA-NEBRASKA 



lowan stage, the occurreiK-e of wliicli is so problematical, especially in 

 western Iowa. 



dlAXXELS OF THE LATER StAGE 

 GENERAL COXDITIONS 



As already stated, the other channels considered belong to the drain- 

 age of a later epoch when the conditions were considerabl}'' changed. 

 The Kansan ice had disappeared and the larger streams were running 

 much as before its advent. For convenience these channels are named 

 after present streams which occupy portions of the old channels. Tliey 

 will be described in the order of their size, namely, the Xiobrara, Ponca, 

 Mosquito, and Choteau. 



THE AyCIENT XIOBRARA 



There will here be considered only the old course from Niobrara, Ne- 

 braska, to the James. Presumably its former course elsewhere was the 

 same as that of the present stream. 



The reasons for believing that the Niobrara formerly crossed the line^ 

 of the present Missouri Yalley east of Springfield and flowed over the 

 divide at Tabor and down Beaver Creek to the James are briefly as 

 follows : 



1. The most obvious evidence is the topography, for the general slopes 

 from the south as well as from the north converge toward the old valley, 

 which has now an altitude of about 1 ,350 feet. The highland west of 

 Yankton rises to altitudes more than 1,500 feet and declines regularly 

 toward the old Niobrara Yalley, and the slope is deeply covered with 

 drift clays. To the south lies tlie gorge of the Missouri, much 5'^ounger 

 in appearance and exposing older I'ocks everywhere. 



2. There are continnons chalk cliffs on botli sides of the Missouri above 

 Yankton excepting for an interval fiJled witli stream (h^posits, extending 

 from a point below Springfield to a little east of Bonhomme. The west 

 bank is well defined, but the east bajdv is less clear, perliaps partly on 

 account of a thicker deposit of till on that side. The interval is about o 

 miles wide, but as the stream may cut the bank obliquely the minimum 

 width is somewhat less. A section of the old river channel deposit in 

 this gap as exposed in a perpendicidai- cliff facing the I'iver is as follows: 



Feet 



Black soil 2 



Dark Imff loess-like silt 12 



Darker soil-like layer 1 



Typical yellowish till 58 



