84 THE ILLINOIS GLACIAL LOBE. 



strongly developed portions of the Pecatonica esker belt, and this has led 

 him to suggest that along the lines where these deposits are heavily devel- 

 op! sd there is the culminating limit of some stage of glacial readvance — a 

 suggestion which implies that the ice had not reached an entirely stagnant 

 condition at the time the eskers were forming. The reader will find in 

 Hershey's discussion of these ridges a detailed account of the several instances 

 of transported rock ledges which he has examined, together with a discus- 

 sion of their probable mode of origin. So far as known to the writer, such 

 remarkable transportation of rock ledges has not been reported in other 

 localities. Possibly the disturbed beds Avhich pass horizontally into and 

 rest upon undisturbed beds, have been wrinkled in post-glacial time in the 

 manner suggested by Gilbert for the somewhat similar phenomena in New 

 York, and which find illustration also in northeastern Ohio. 1 



GLACIAL STRLE. 



Stria? have been found not only in the interior jDortion of the district 

 covered by the Illinois lobe but also in several instances near the extreme 

 borders. The latter usually bear directly toward the drift border. Thus 

 in western Illinois the bearing is westward, in southwestern Illinois south- 

 westward, in southern Illinois southward, and in western Indiana southeast- 

 ward, as appears on the glacial map, PI. VI. The striae in southeastern Iowa 

 present an apparent exception, there being several eastward and but one 

 westward bearing observed. But it should be remembered that that district 

 was covered prior to the Illinoian invasion by another ice lobe which 

 deployed eastward across southeastern Iowa, and it is to that ice lobe that 

 the eastward-bearing stria? are referred. 



There are wide areas in this region where striae have not been discov- 

 ered. The absence of observations does not imply that the ledges are not 

 striated, for too little attention has been given the rock surfaces to insure a 

 complete mapping. It is probable that many more instances will be 

 reported as detailed investigations are carried on. There are, however, 

 extensive areas in which no rock exposures occur, and other areas in which 

 the surface rocks are of such character as to break down quickly upon 

 exposure, so that stria? can scarcely be preserved. This condition is found 



'Proc. Am. Assoc. Adv. Sci., Vol. XXXV, 1886, p. 227. Also Vol. XL, 1891, pp. 249, 250. (See 

 also Bull. Geol. Soe. Amer., Vol. X, 1899, pp. 131-134.) 



