DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 705 
the slopes north of the state line and here also Rock River cut down 
into the Potsdam sandstone, so that the uplands of Rock and Green 
counties west of Rock River were much dissected and almost wholly 
reduced to slope. East af Rock River, however, the southeasterly 
dip depressed the St. Peter sandstone in the east slope of the valley and 
carried it wholly below the bottom of the pre-glacial valley which 
bordered the eastern upland on the east in Walworth and McHenry 
counties. ‘This confined the erosion more largely to the horizon of the 
limestones and as a result there was less dissection east of the river and 
much of the upland plain was left. Similar conditions resulted from 
the southward dip south of Pecatonica River on the west side of the 
Rock Valley. The St. Peter sandstone comes up again locally between 
Byron and Dixon, Illinois, but here other relations control the final 
result. The deposition of the drift sheet under discussion mantled 
these slopes and uplands but, as has been shown, the pre-glacial 
topography was only masked, not wholly obliterated, so that subse- 
quent drainage for the most part followed earlier courses. . Thick 
deposits were preserved in the valleys, but in the more dissected 
tracts, especially where the upper slopes were steep, conditions have 
been favorable for the accomplishment of considerable erosion. 
Not only did the character of the pre-glacial topography affect 
erosion of the drift, but the relocation of Rock River, the master 
stream, which followed the retreat of the ice front had also some 
effect. When the glacial margin was melted back to the east side of 
the pre-glacial Rock Valley above the mouth of Kishwaukee Valley 
south of Rockford, the accumulated waters from the blocking of the 
drainage and melting of the ice found outlet across a series of cols and 
along the intervening valleys between Rockford and Sterling, so that 
the post-Illinoian stream became located along a new course, that 
which is now followed between Rockford and the Mississippi. In 
deepening its valley along this line the river was forced to cut gorges 
in limestone and sandstone at several points. ‘There is some evidence 
that this cutting began 100 feet or more above the level of the present 
stream. Just how much below this level these gorges were cut before 
the outwash gravels of the Wisconsin stage came in is not known, 
but it is evident that their erosion must have retarded the work of the 
main stream and its tributaries throughout the area under discussion. 
