288 The American Geologist. May, 1896 
This latter is the old interglacial sojl of the Aftonian epoch, 
and presents features characteristic of that stage of the Gla- 
cial period. After having thoroughly compared it with the 
ancient soil developed all over northwestern Illinois, and hav- 
ing traced the direct continuity between the two, it is impos- 
sible to doubt its nature and age. It is essentially a till or 
boulder-clay with a small percentage of sand and a few scat- 
tered subangular pebbles and small boulders of foreign deri- 
vation and a somewhat larger quantity of angular white chert 
fragments. This deposit has been oxidized, leached, and 
darkened in color, and has so peculiar characteristics that it 
can be distinguished at a glance from all other formations of 
the region. 
No. 3 of the section is the till or boulder-clay, or, as it is 
frequently called, "hard pan," of the Kansan drift sheet 
(unmodilied, except by a certain amount of oxidation and 
leaching consequent on its having formed for a long time the 
subsoil of the Aftonian interglacial soil). It is here a stiff, 
light yellowish gray clay, presenting on very fresh surfaces a 
slight appearance of an irregular laminated structure, pro- 
duced probably by the pressure of the ice. as it is a portion of 
the ground moraine or subglacial drift; or perhaps this struc- 
ture may be due to the subsequent action of percolating wa- 
ters. At this place the deposit contains comparatively few 
pebbles (among which, however, I have found some beautifully 
striated) ; but on tracing it along the bluff to some distance, 
it is found to abound in glaciated foreign rock fragments, 
and to be as characteristic a till as any in America. I have 
dwelt thus long on this least important portion of the section 
to show that there can be no reasonable doubt of the early 
age of the lower strata here exposed. 
When the ice first invaded the region about Freeport, it 
found a moderately rolling surf ace covered with a prevailingly 
red clay soil abounding in white angular chert. This red 
stony clay was partially frozen before the ice reached it, and 
at many places escaped a complete removal, still remaining 
under the drift sheet as isolated remnants of a once continu- 
ous mantle. The 8 inches of red clay and chert (No. 4 of the 
section) are a portion of this ancient preglacial soil, which 
has been eroded by the ice from a slight ridge of rocks now 
