372 I~tinois NaturAL History SurvEY BULLETIN 
corded extremes of temperature are —35 
degrees at Mount Carroll (January 22, 
1930) 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 western 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 32.3 at Mount Carroll. The wettest 
month in northern and west-central IIli- 
nois is June. The wettest month in south- 
ern Illinois varies with location; May 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, lowa, weather station), 66.77 per 
cent. 
Frost.—The average frost-free periods 
along the western border of Illinois are 
April 19 to October 16 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 prevailing wind in winter 
along the western border of Illinois is 
from the northwest; in summer, it is from 
the southwest, often hot and dry. 
Physiography 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 bluffs on which hill 
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 
Vol. 26, Art. 5 
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 surficial 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.—Surficial 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. On most other prairie slopes the 
surficial 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 & Willman 
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 
