700 WILLIAM C. ALDEN 
chunks, and the calcareous constituents are almost entirely removed. 
Where greater thicknesses are exposed, such as 5 or 10 feet or more, 
a very striking condition is revealed (Fig. 6). The body of the drift is 
found to be fresh, slightly altered, highly calcareous, clayey till, of 
light buff, pinkish, bluish, or grayish tint, in which the crystalline 
pebbles are sound with the exception of some of the diorites, or more 
basic granites. Fully 80 per cent. and often 85 to go per cent. of the 
pebbles in this unaltered part of the drift are limestone and these are 
Fic. 4.—Illinoian upland drift plain of remarkably long, gentle, uneroded slope. 
The drift is probably of moderate thickness, overlain by a few feet of loess and underlain 
by Galena limestone. ‘Three miles northeast of Seward, IIl. 
undecayed and have smooth, clean surfaces, often highly polished 
and delicately striated. Within two or three feet of the top of the 
till and the base of the loess, however, there is an abrupt change in 
character and appearance. Passing upward the drift becomes dark 
brownish in color, the clayey matrix is found to be nearly or quite 
leached of its lime carbonate, and the few limestone pebbles which 
remain are either so rotted as to crumble between the fingers or the 
surfaces have been roughly etched by solution. In places only a soft 
yellow powder marks the former position of a limestone pebble. 
Within a few inches this grades upward into a dark-red layer from 
