44 THE ILLINOIS GLACIAL LOBE. 



valleys which had eastward drainage in preglacial times and are probably 

 to be attributed to the ponded waters held in front of the ice and laden 

 with large amounts of fine sediment carried by the waters issuing from the 

 ice. Portions of the border in northwestern Stephenson, southeastern Jo 

 Daviess, and northwestern Carroll counties are liberally strewn with bowl- 

 ders of granite and other distantly derived rocks. The number appears to 

 be greater within the first 5 miles back from the drift border than at more 

 remote points. In this portion of the drift border the loess coating is thin 

 except in the immediate vicinity of the Mississippi Valley, its average depth 

 being scarcely more than 5 feet. On slopes it is largely removed, leaving 

 the surface of the glacial drift exposed to view. 



The interval between the southern point of the Driftless Area near 

 Savanna and the northernmost point at which the Illinoian drift border is 

 recognized on the Iowa side of the Mississippi is about 50 miles. A direct 

 line across it traverses a low plain covered with a very bowldery sheet of 

 Iowan drift, described by McGee, which is nearly free from deposits of 

 loess in the middle portion, but which is bordered in the peripheral portion 

 on the north east and south by loess-covered drift. No recognition of the 

 Illinoian drift has been made in this interval along the direct line of con- 

 nection. But it has been identified in Davenport and at points west of that 

 city in Scott and eastern Muscatine counties, Iowa, It is therefore certain 

 that the Illinois lobe extended beyond the Mississippi River at least as far 

 north as eastern Scott County. 



Prof. J. A. Udden has recently published an important table showing 

 notable differences in the rock constituents of the Illinoian and the under- 

 lying drift sheets of Muscatine County, from which it appears that the 

 constituents of the Illinoian are largely derived from outcrops to the eajst. 1 



Exposures of drift in Davenport and Muscatine, Iowa, were made the 

 subject of joint investigation by Prof. Samuel Calvin and Dr. H. Foster 

 Bain, of the Iowa Geological Survey, Prof. J. A. Udden, and the writer, in 

 November, 1897, and there was entire unanimity in the interpretations. 

 At Davenport the first exposure examined was one previously described by 

 McGee, 2 which is situated at the northwest corner of Sixth and Harrison 



1 Iowa Geol. Survey, Vol. IX. 1899, p. 336. 



- Eleventh Ann. Rept, U. S. Geol. Survey (for 1889-90), 1891, p. 491; also tig. 77. 



