556 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



river which now empties into the Missouri river 75 miles above 

 Sioux City formerly flowed through the Onawa channel or 

 formed the Old Moing-ona stream is something of a moot ques- 

 tion. The headwaters of the last mentioned w^aterway might 

 have been any one of the three important streams rising in the 

 Black Hills — the Cheyenne river, the White river, or the Nio- 

 brara river. Of course until the first great ice sheet advanced 

 the iMissouri river below the Mandan was non-existant and its 

 headwaters flowed into Hudson Bay as already mentioned. As 

 the continental glacier spread southward it necessarily cut off 

 the discharge of this river and diverted it. As Professor Todd 

 astutely surmises the IMissouri waters probably were empounded 

 by the ice until they overflowed the rim of its basin and then 

 emptied into the first eastward flowing stream to the south, at 



Fig. 176. — Relative magnitudes of present Des Moines River and old Moin- 

 gona River. 



the same time cutting a gorge along the glacier front. This 

 process was repeated until the river had excavated its way to 

 the Kansas river and the ice had covered the lower channels of 

 all of the streams running eastward from the Rocky Mountains. 

 The excessive thickness of the Glacial till east of Sioux City 

 and along the paths of the two old river valleys under consider- 

 ation has been long known. Singularly enough Dr. H. P. Bain, 

 who has done considerable work on the Glacial deposits of this 

 portion of the state, mistook the more northerly one of the 

 thickened belts for the terminal moraine of the lowan Drift 

 Sheet.^ Later when he came across the thickened belt of the 

 Onawa channel he regarded this as the real southern limit of the 

 lowan drift.** The fact that these thickened belts were not above 



»Iowa Geol. Survey, Vol. VI, p. 462, 1897. 

 »Iowa Geol. Survey, Vol. VIII, p. 351, 1898. 



