4 FRANK LEVERETT 



excavate in postglacial time the entire valley, which it now but 

 partially occupies. 



A few years later General G. K. Warren discovered the aban- 

 doned section of the preglacial valley which crosses Lee county, 

 Iowa, a few miles west of the lower rapids, and connects the 

 portion occupied by the stream above the rapids with that 

 below. In his report in 1878 he presented a discussion illus- 

 trated by a map setting forth the position of the old channel. 1 

 General Warren based his interpretations upon the absence of 

 rock outcrops in the valleys which traverse the old course of the 

 river, there being no borings that extended to the rock bottom. 

 A few years later a boring at Mont Clare, la., was sunk in the 

 old valley and brought confirmation to General Warren's inter- 

 pretation. 2 The accompanying sketch map, Fig. 2, sets forth 

 the position of the old valley and its relation to the one across 

 the rapids. 



It should not be inferred that this broad preglacial valley 

 was necessarily a line of discharge for the whole of the present 

 drainage basin of the upper Mississippi. The available evidence 

 concerning the preglacial drainage, though imperfect, is thought 

 to indicate that a large part of the region above the upper rapids 

 may have drained southeastward through the Green River Basin 

 to the Illinois. Hershey has suggested a northward discharge 

 for the headwater portion of the basin, a suggestion which awaits 

 adequate investigation. 3 The preglacial valley which passes the 

 lower rapids on the west is nearly coincident with the present 

 Mississippi from the head of these rapids up to Muscatine, but 

 its position farther north has not been ascertained, nor has the 

 size of its drainage basin been even approximately determined. 

 It is probable, however, that much of eastern Iowa was tribu- 

 tary to this preglacial line. 



'Report of the U. S. Army Engineers for 1878-9, Vol. IV, Part 2, pp. 916, 917, 

 Diagram E ; also Diagram I, Sheet 4. 



2 Buried River Channels in Southeastern Iowa, by C. H. Gordon, Iowa Geol. 

 Survey, Report for 1893, pp. 239-255, Figs. 5, 6, and 7. Published in 1895 as Vol. 

 Ill of the present survey. 



3 American Geologist, Vol. XX, 1897, pp. 246-268. 



