296 
The American Geologist. 
May, lbUG 
level. Our region, once greatly elevated to admit the excav¬ 
ation of the deep Quaternary valleys, had again subsided to a 
comparatively low level in the Kansan epoch, although still 
higher than at present. But we have no evidence that, prev¬ 
ious to the formation of the Kansan drift-sheet, the land had 
sunk to so low a level as to permit the streams of northwestern 
Illinois to form a flood-plain, reaching to at least fifteen feet 
above the present food-plain level. On the contrary, evidence 
adduced in other regions, as well as here, indicates that the 
land stood, from the time of the great elevation to the Kansan 
epoch, continuously higher than now. Consequently, although 
the silt under discussion presents a few features slightly 
favoring a fluvial origin, it can not be a river-swamp deposit. 
Obstruction of the Pecatonica valley by westwardly moving- 
ice has repeatedly produced a lake in the valley west of the 
ice-front. We shall now consider the hypothesis that the silt 
was due to that cause. It bears such a strong resemblance to 
certain loess deposits that glacialists who have seen it have 
promptly pronounced it “a buried loess.” Its composition is 
essentially that of a glacial deposit; but there is nowhere in 
the Pecatonica basin any rock formation that could have sup¬ 
plied the material for its formation. Besides, as this region 
had not yet been glaciated, the derivation of its evidently 
glacial material must have been from some region on the east. 
Since no silt could be carried westward in the valley except 
when occupied by a lake through the obstruction of the mouth 
by ice, we thus have evidence that such a lake existed earlier 
than the lake Pecatonica already described, and that the ice- 
front then stood, as again later, on some line west of the 
Rock river. 
This entire deposit was originally of a uniform deep blue 
color, due principally to the variety of iron oxide which it 
contained. Not only is this color chiefly associated with gla¬ 
cial deposits, but it seems to be characteristic of clays and 
silts of cold climates. I have mentioned that particles of 
carbon are disseminated through the deposit. These indicate 
that the waters came in contact, somewhere in their course, 
with vegetation. Small broken branches of trees have been 
found, but are very rare. The nature of the vegetal remains 
is not such as to indicate the existence, on the neighboring 
