16 The American Geologist. January, 1895 
taining also about the same thickness thi'oughout all the 
northern half of Stephenson county. There was apparently 
10 feet of Upland loess in Freeport and its vicinity ; and from 
here it thickens toward the west and southwest, so that it 
may have amounted to 20 feet in the southwest corner of the 
county. The thinning toward the east is in contrast with the 
Valley loess, and seems to indicate that the main source of 
the sediment lay beyond or up the Mississippi valley, instead 
of eastward as at the time of deposition of the Valley loess. 
The Upland loess is everywhere of greatest thickness in the 
vicinity of the Mississippi river. 
A few shells have been observed in this formation at a few 
localities, but they are rare and not of much importance. In- 
deed, the loess-depositing waters in the Pecatonica and Rock 
river valleys must have been nearl}' lifeless, in marked con- 
trast with the abundance of invertebrates in the streams of 
the preceding Florence subepoch. 
It has been concluded from a study of the piienomena con- 
nected with the Upland loess that it was deposited under al- 
most purel}^ lacustrine conditions. The sediment gradually 
subsided to the bottom of the great lake where it was laid 
down with neither current nor wave action. The lake must 
have had a depth, over the lower portion of the Pecatonica 
valley, of at least 250 to 300 feet; over the Mississippi valley 
its depth was 600 feet or more; and throughout the greater 
part of northwestern Illinois it exceeded 100 feet. 
The different beds or divisions of the loess of this district 
were deposited one after the other without ^ny stratigraphic 
break between them. Their phenomena indicate that they 
were formed in a lake, or series of lakes, at first long and nar- 
row, with many similarly shaped arms, and having strong 
currents capable of carrying coarse sand ; but later gradually 
increasing in depth and width, with a consequent decrease in 
the sediment-carrying power, rising slowly along the valley 
slopes and over the upland ridges, until finally perhaps the 
highest land in northwestern Illinois was covered with water. 
This gradual increase in the size of the lake must have been 
brought on by a gradual subsidence of the land, which, as we 
have seen, began previous to the formation of the lowest por- 
tion of the loess. 
