374 
higher temperatures. Such slopes, espe- 
cially those of high bluffs, are also directly 
exposed to the prevailing southwesterly 
WISCONSIN GLACIAL 
SANGAMON INTERGLACIAL 
BUFFALO HART 
JACKSONVILLE 
PAYSON 
LOVELAND (PRO-ILL.) 
ILLINOIAN GLACIAL 
YARMOUTH INTERGLACIAL 
KANSAN GLACIAL 
AFTONIAN INTERGLACIAL 
NEBRASKAN GLACIAL 
Fig. 4.—Classification of geologic time dur- 
ing the Pleistocene Period. (After Leighton & 
Willman 1950.) 
B PROFILE 
SECTION C-C 
ILtinois NATURAL History SURVEY BULLETIN 
Vol. 26, Art. 5 
winds of the growing season, which in- 
crease the water-loss both from plants and 
the surface soil. The upper bluff slopes 
are consequently extremely xeric and sup- 
port prairie rather than forest. 
Where the bottomlands are wide, wind 
movement is unimpeded for some dis- 
stance. Brow slopes of bluffs bordering 
such bottomlands are xeric. Accordingly, 
hill prairies are common in the northern 
part of the American Bottoms in Madison 
and St. Clair counties where the Missis- 
sipi River valley is wide. In northern 
Cass County, hill prairies are found on 
bluffs that trend almost east-west and 
that border a broad, flat, sandy terrace 
that extends to the Illinois River, which 
here flows southwestward. The xeric con- 
ditions on these brow slopes result in part 
from the movement of westerly winds 
over the broad, flat, sandy terrace and in 
part from the deep mantle of very sandy 
loess, which favors rapid loss of water by 
downward percolation. 
LEGEND 
Cj) UPLAND 
ES] SLOPE 
VALLEY 
Fig. 5.—Schematic plan, 4, and profile, B, of a bluff of type common along major stream 
valleys in Illinois. The steep valley wall between the two large tributary valleys in the plan is 
the bluff shown in the profile (section C-C). The bluff includes a talus or toe slope, a cliff, 
a rock ledge, and an upper or brow slope, which in the profile shows a cover of prairie. 
