372 



Illinois Natural History Survey Bulletin 



Vol. 26, Art. 5 



corded extremes of temperature are -35 

 dej;;rees at Mount Carroll (January 22, 

 I'^'.^O) and 115 degrees at Greenville 

 (July, 1936). The highest recorded tem- 

 perature in that part of Illinois along the 

 Mississippi River is 114 degrees, recorded 

 at Quincy in July, 1936. 



Precipitation. — The average annual 

 precipitation along the \\estern border of 

 Illinois varies from 33.13 inches in 

 northwestern Illinois (Dubuque, Iowa, 

 weather station) to 41.39 at Cairo and 

 47.43 at Anna. Averages of snowfall, 

 notably less than 10 per cent of the an- 

 nual precipitation, are 9.9 inches at Cairo 

 and M.,^ at Mount Carroll. The wettest 

 month in northern and west-central Illi- 

 nois is June. The wettest month in south- 

 ern Illinois varies with location; Ma\' is 

 the wettest month at Anna, March the 

 wettest at Cairo. During the growing 

 season, April through September, Cairo 

 receives 49.41 per cent of its annual rain- 

 fall ; Anna, 52.77 per cent; Quincy, 65.26 

 per cent; and northwestern Illinois (Du- 

 buque, Iowa, weather station), 66.77 per 

 cent. 



Frost. — The average frost-free periods 

 along the western border of Illinois are 

 April 1*^ to October Ifa in northwestern 

 Illinois (Dubuque, Iowa, weather sta- 

 tion), April 13 to October 20 at Quincy 

 in west-central Illinois, April 9 to Octo- 

 ber 26 at Anna, and March 30 to Octo- 

 ber 29 at Cairo. 



Wind. — The pre\ ailing wind in \\ inter 

 a'ong the western border of Illinois is 

 from the northwest ; in summer, it is from 

 the southwest, often hot and dry. 



Ph>siograph>- and Geology 



Steep slopes or bluffs abut the broad, 

 deeply alluviated floodplains or bottom- 

 lands of many of the major stream valleys 

 in Illinois. The continuity of the bluffs 

 is broken by tributary streams that enter 

 the main valleys. The underlying bed- 

 rock and the surficial material determine 

 the form of the bluffs. 



The bedrock of the blufts on \vhich bill 

 prairies occur is limestone, dolomite, sand- 

 stone, shale, or combinations of these; the 

 geological ages of the uppermost strata 

 range from Middle Ordovician to Upper 



Pennsylvanian. In most places, the bed- 

 rock crops out to form a cliff as much as 

 200 feet above the valley floor. Fre- 

 quently, a stony talus or toe slope is pres- 

 ent at the base of the cliff, fig. 2, and, un- 

 less recently disturbed, supports a mixed 

 forest. A mantle of surlicial material cov- 

 ers the bedrock at the top and forms the 

 brow or upper slope of the bluff. In some 

 places, as in parts of Morgan and Madi- 

 son counties, sandy loess and colluvium 

 completely mantle and conceal the bed- 

 rock. 



Surficial Material. — Surlicial mate- 

 rial, the unconsolidated material above 

 the bedrock, may be residual or trans- 

 ported. Residual material, which sup- 

 ports both prairie and forest in Illinois, 

 occurs south of the glaciated area, as, for 

 instance, on the rocky slopes at Cave 

 Creek prairie and the cherty ridge-top at 

 Tamms. (^n most other prairie slopes the 

 surlicial material is transported — loess or 

 glacial drift. 



Loess, a windblown accumulation of 

 silt with subordinate clay and minor 

 amounts of fine sand, occurs over large 

 areas of the Midwest. "One of the most 

 important and best-known occurrences of 

 loess in the world is in the Mississippi 

 River Basin" (Leighton iSc Wi'lman 

 1950). Loess mantles most of Illinois ex- 

 cept the large stream valleys and areas of 

 lake sediments and sand dunes. 



The eolian hypothesis of the origin of 

 loess deposits is the one accepted by most 

 geologists who have studied this material 

 extensively. Udden (1894) wrote, "From 

 a dynamical point of view the wind-the- 

 ory would appear to furnish an adequate 

 explanation of the occurrence of the loess 

 in the Mississippi valley, at least as to 

 most of its phases." Shimek (1896) ad- 

 vocated the eolian hypothesis and based 

 his conclusions in part upon the land snail 

 shells he found in loess. Chamberlin 

 (1897) presented the hypothesis that loess 

 (of the Mississippi Valley) is a wind de- 

 posit and that the sources of the material 

 were the floodplain deposits of the glacial 

 rivers. The hypothesis of the origin of 

 loess deposits from backswamp sediments 

 (Russell 1944) does not seem tenable for 

 Illinois. 



The thickest loess deposits in Illinois 



