374 



Illinois Natural History Survey Bulletin 



Vol. 26, Art. 5 



higher temperatures. Such slopes, espe- 

 cially those of iiigh hluffs, are also directly 

 exposed to the prevailing southwesterly 



Fig. 4. — Classification of geologic lime ilur- 

 iiig the Pleistocene Period. (After Leighton & 

 Willman 1950.) 



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. Hat, sandy terrace 

 that e.xtends to the Illinois River, which 

 here flows southwestward. The xeric con- 

 ditions on these brow slopes result in part 

 from the movement of westerly wind> 

 over the broad, flat, sandy terrace and in 

 part from the deep mantle of very sandy 

 li/css, which favors rapid loss of water by 

 (knvnward percolation. 



UPLAND 



PRAIRIE SLOPE 



B PROFILE 



iP^'^'^'-^^ SECTION C-C 



PLAN 



LEGEND 

 □ UPLAND 

 ES SLOPE 

 CH VALLEY 



Fig. 5. — Schematic plan, .-/. ami 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, \vhich in the profile shows a cover of prairie. 



