IOWA. ACADEMY OF SCIENCES. 



of the river, there being no borings that extended to the rock 

 bottom. A few years later a boring at Mont Clare, Iowa, was 

 sunk in the old valley and brought confirmation to General 

 Warren's interpretation." The accompanying sketch map, 

 figure 2, sets forth the position of the old valley and its rela- 

 tion to the one across the rapids. 



Figure 1. Sketch map of regloQ discussed, showing- cou^'se of old channel?. 

 Note of Explanation.— The abandoned portion of the pre-glacial channel of the 

 Mississippi is shaded. Hachures are med to Indicate valley borders b.)th below and 

 above high terraces and along the temporary Mississippi channel, opened at the 

 Illinoian stage of glaciation. Thd extent of the high terrace in Missouri has not been 

 determined. 



It should not be inferred that this broad, preglacial valley 

 was necessarily a line of discharge for the whole of the pres- 

 ent drainage basin of the upper Mississippi. The available 

 evidence conceraing the preglacial drainage, though imper- 

 fect, is thought to indicate that a large part of the region 



* Burled River Channels in Southwestern Iowa, by 0. H. Uordon, Iowa Geol. Surv. 

 Ileport for 1893. pp 236-255. Figs. 5, 6 and 7. Published in 1895 as Vol. Ill of the present 

 survey 



