Erosion Cycles in XorfJnres/eni Illinois. — Hershey. 83 
At the close of cycle No. 3 northwestern Illinois was a 
gently rolling plain. The general upland surface was about 
100 feet above the broad plainlike vallej^s of the larger streajiis. 
We cannot say that the sea coast during this cycle of erosion 
was more distant than during the preceding one; and, as the 
larger valleys were a perfect baseleveled plain, we will assume 
the same drainage gradient for the streams of this cycle. As- 
suming the ditference in altitude of the two peneplains to be 
the actual amount of uplift, we obtain a maximum altitude for 
our district at the end of cj'^cle No. 3 as 5^1 feet. The average 
upland altitude would be 1266 feet above the sea level of that 
time. It is possible that our estimates for this cycle are too 
low, as the streams, meandering on the broad flood-plains of 
the valleys, were doubtless much longer than the basins in 
which they flowed. 
B((selei-el No. .!,. The present valleys of the streams of 
northwestern Illinois are comparatively narrow, flat-bottomed 
trouglis. The sides are frequently very steep, occasionally 
precipitous walls of bare rock. These blutfs vary from a hight 
of 80 feet in the lower Fecatonica valley to 250 feet along the 
Mississippi river. There are 200-foot bluffs along the Rock 
river between Grand de Tour and Oregon, but the stream is 
out of its old course and these bluffs, are not preglacial, so that 
they do not aff'ect the present discussion. The width of these 
valleys varies according to {(i) the size of the stream flowingin 
them, {b) tlie nature of the rock excavated and (c) the depth 
of the gorge. The soft Cincinnati shales give broad valleys 
with gentle slopes ; the Galena limestone permits much nar- 
rower valleys, with distinct bluff's and mural precipices; the 
Niagara limestone forms slightly narrower but otherwise sim- 
ilar valleys ; and the St. Peter sandstone has much narrower 
gorge-like valleys, with steep bluff's but rarelj^ precipitous 
cliffs. The moderately resistant properties of the St. Peter 
sandstone in Illinois are tindy illustrated in the Elk Horn 
creek valley in Ogle county, where a fault witi) a throw of 40 
feet crosses a small valley. On the upstream side (if the fault 
the valle}' is excavated into (ialena and 'I'renton limestones 
and is broad and open; hut, where the fault brings tlie St. 
Peter sandstone above tiie v;iney plain, it suddenly narrf)ws 
to a gorge less tlian oiic-thii'd as wid.- as it is aliove the fault. 
