182 THE ILLINOIS GLACIAL LOBE. 



probable that the loess on the immediate borders of the Pecatonica lobe is 

 water-bedded, even at levels 800 feet or more above tide. Possibly this 

 lobe held a body of water in the Pecatonica Basin at a level nearly as high 

 as the bordering' uplands (900 feet above tide). 



The relation of the bluff loess to the upland loess is a question of prime 

 importance. It has been commonly assumed that deposition was completed 

 at nearly the same date on the uplands and on the valley borders, and that 

 the difference in porosity is due to a greater strength of the current along 

 the line of the main valleys. It is probably true that the main valleys 

 were the line of strongest current during the deposition of the upland loess, 

 and possibly the deposition of the bluff loess was completed at a date nearly 

 as early as that of the water-laid deposits on the bordering uplands. There 

 is thought, however, to be evidence pointing to a continuance of loess depo- 

 sition along the main valleys after deposition had practically ceased on the 

 uplands. The evidence referred to consists of an excessive filling by loess 

 of the valley recesses and lower courses of tributaries, such as seem explain- 

 able as a result of transportation down the valley after the flooding of the 

 uplands had ceased. Accumulations of this class are especially noticeable 

 on the lower course of the Wabash and Ohio rivers, and they are found to 

 some extent along the Illinois and Mississippi. Mr. H. F. Bain also reports 

 that the same feature is noticeable along the borders of the Missouri River 

 in western Iowa. 1 It is found that the blocking is most conspicuous on the 

 east side of the valleys, a feature which suggests that wind action has been 

 effective in causing the blocking of the mouths of tributaries, for the pre- 

 vailing winds are from the west. Upon the retirement of the glacial waters 

 to the limits of the main valleys or to their immediate borders the material 

 available for transportation would be chiefly loess, and the loess would have 

 had little opportunity to become leached. We may suppose, therefore, that 

 a deposition of unleached loess continued until the glacial waters had retired 

 completely and the streams were fed only by the rainfall of the region. 

 This may have been maintained for some time after the disappearance of 

 the Iowan ice sheet, though it appears not to have continued sufficiently long 

 to have overlapped to any marked degree the valley excavation which 

 followed the loess deposition. 



As noted above, the fossils of the loess are confined largely to the 



1 Discussion at Twelfth Annual Meeting Iowa Acad. Sci., Dec, 1897. 



