702 WILLIAM C. ALDEN 
material as comprising but two-thirds of the unaltered drift the 
amount of alteration accomplished is considerable in reducing a thick- 
ness of g to 12 feet to 3 or 4 feet of residuum. ‘This is much greater 
than the amount of alteration of the Wisconsin drift and indicates a 
much longer time of exposure, a length of time seemingly out of har- 
mony with the amount of erosion which has been accomplished in this 
part of the area—for this residual drift is well developed where the 
Fic. 6.—Exposure of Illinoian drift one-half mile south of Seward, Ill. The 
section shows loess 14 feet—dark-red, residual, weathered Illinoian till from which all 
limestone pebbles and finer calcareous material have been removed by solution, 2 to 3 
feet—buff calcareous till with abundant pebbles of Niagara and Galena limestone. 
The length of the hammer, 16 inches, shows the zone of gradation from residual to 
unaltered calcareous till. 
plains are least dissected by erosion. We have thus the combination 
of rather old drift and very young topography. 
Added to these confusing conditions, one examining this tract west 
of Rock River in Illinois, especially along the borders of the upland, 
and in that part west and northwest of Rockford, frequently finds 
unaltered till coming right up to the surface or to the base of the thin 
coating of loess or loam, or else the unoxidized till has only the cal- 
careous material of the clayey matrix leached out to depths of one or 
