DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 7ol 
which every particle of limestone including the pebbles has been 
removed, leaving only a small amount of sticky red clay, the residuum 
of complete disintegration of the limestone, binding the insoluble 
constituents, sand grains, quartz, quartzite, chert, and dense, fine- 
grained crystallines into a compact gritty mass. This layer is usually 
about 2 feet, sometimes 3 feet in thickness, and the change from 
the unaltered drift below takes place with remarkable abruptness 
generally within less than a foot—often within the space of a few 
Fic. 5.—Illinoian upland drift plain, five miles west of Rockford, Ill. Galena 
limestone covered with 1 to 40 feet of drift. There are here extensive tracts of upland 
and long gentle slope but little affected by erosion. 
inches. The red till is not a distinct deposit, for the change is 
usually gradual, though occurring within so small a distance. If 
the high percentage of calcareous material present in the unaltered 
part of the till is supposed originally to have continued to the surface 
of the deposit, as there is every reason for thinking it did, then this 
residual layer of insoluble constituents must represent but 20 per cent. 
of the thickness subjected to alteration. In other words, the 3 or 4 
feet of residual till represents the concentration of the insoluble con- 
stituents in an original thickness of 15 or 20 feet of unaltered till. 
Even if for conservatism we consider the calcareous and other soluble 
