Pleistocene Deposits of Illinois. — Hershey. 299 
stratification or interlocking of colors. The cause which 
changed the color operated deeper in the coarse lenses of silt 
than in the finer, and crept into the interior from cracks and 
vertical lines of weakness in irregular tooth-like projections, 
producing an effect which is common in the calcareous shales 
of the Cincinnati strata on the “mounds” of this region, and 
probably due to the same cause. On following the brown 
layers back into the hill, they are found to change into the 
blue; but still a certain thickness of brown silt remains at 
the surface. 
The silts are horizontal, wdiile the variegated clays rest un- 
conformably on them, sloping toward the center of the valley, 
and coming successively in contact with lower layers of the 
silt. This is an erosion unconformity. As the deposition of 
the overlying clays preceded the first incursion of the ice- 
sheet in this vicinity, glacial abrasion was not the cause of 
the erosion, but the removal of the silt was effected under the 
ordinary agencies of subaerial and stream erosion. Now it is 
important, in order to determine the length of the erosion 
interval here indicated, to know approximately what amount 
of material was removed. Unfortunately, our exposure is so 
limited that we can do little more than guess at it. Probably 
the horizontal layers of silt, now exposed, originally extended 
far out into the valley, perhaps even across it. If this was so, 
the amount of material removed was large. Again, any deposit 
such as this, which could have accumulated to a thickness of 
20 or more feet in a position not more favorable to sedimen¬ 
tation than any other, and at a hight altogether above the 
present stream level, must have been extensively developed 
all along the valleys of the region. Yet it has been almost 
entirely removed above the present stream and to a consider¬ 
able depth below it. As the frequent presence of the variega¬ 
ted clays shows, the ice-sheet effected very little abrasion at 
low levels; and this fact, with the absence from the till, west 
of the valleys at Freeport, of any considerable amount of 
material that could have been derived from the blue silt, 
makes it certain that the removal of this blue silt not only 
preceded the arrival of the ice-sheet, but also antedated the 
deposition of the variegated clays. At any rate, the amount 
of material removed during this erosion interval was consider- 
