&54 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



had melted or retreated the Missouri river had established a 

 permanent course as it exists today, and could not get back to its 

 orig-inal path. This is essentially the idea advanced by Prof. 

 J. E. Todd.« 



The main problem concerning the course of the ancient Moin- 

 gona river is to establish some intimate connection between the 

 old stream course where it unites with the present Des ]\Ioines 

 river valley and some of the Rocky Mountain rivers which still 

 follow their original Tertiary paths. This is accomplished by 

 restoring in fancy the bed-rock surface beneath the drift mantle 

 of the State. In this respect the published data are now very 

 complete. The multitude of well sections published by Prof. 

 W. H. Norton" form only a sm'all part of the available infor- 

 mation. Especially along the line of the present investigation 

 there is a large supplement of personal records. Comparison 

 and adjustment of these data indicate a pre-Glacial surface of 

 the State that is very much more rugged and broken than that 

 of the present. The old inequalities of relief are now largely 

 smoothed over by the till. Topographically much of the eastern 

 half of the State is evidently 'not very unlike the driftless area 

 of the extreme northeastern Iowa. The western half of the 

 State is manifestly not nearly so rough. 



In western Iowa the relief that existed at the end of Tertiary 

 time and immediately before the first invasion of continental 

 ice presents extreme differences in altitude of between 400 and 

 500 feet. Over the old elevations the drift is often scarcely a 

 score of feet in thickness. In some of the old depressions and 

 valleys the Glacial drift is 300 to 500 feet thick. The disposi- 

 tion of the low places is such that they lie in long belts and 

 old gorges having relatively steep sides. Some of these primi- 

 tive troughs are manifestly the paths of extinct streams. The 

 one so well known at Des IMoines is now known to extend far 

 beyond that neighborhood. Its narrow belt is traceable north- 

 westward to Sioux City where it intersects the present INIissouri 

 Valley. Throughout its entire length the course of this great 

 one-time stream is well defined. Between Des Moines and Sioux 

 City the mean fall is slightly above two feet to the mile— about 

 the same as that of the Des Moines river of today between the 

 Raccoon Fork and its mouth. Curiously enough the depth of the 



"Science, N. S.. Vol. XXXIX, pp. 263-274. 1914. 

 Uowa Geol. Survey, Vol. XXI, 1912. 



