394 
and geology. In making deductions con- 
cerning plant succession and vegetational 
history of Illinois hill prairies, it is nec- 
essary to keep in mind some of the Pleis- 
tocene history of the area in which these 
prairies occur. 
The strip of land along the present 
Mississippi River valley in western IIli- 
nois has had a vegetational cover since 
Kansan glacial times, fig. 4, except for 
the portion from southern Carroll Coun- 
ty to northern Adams County, inclusive, 
which was covered by ice during part of 
Illinoian glacial time. Except for a strip 
between Fulton, Whiteside County, and 
Cordova, Rock Island County, which was 
covered during Tazewell glacial time 
(Shaffer 1954), the entire western border 
has supported a plant cover since Sanga- 
mon interglacial time. It was during 
Tazewell time that the present valley of 
the Mississippi from the Rock Island area 
south to Adams County was established. 
The eroding valley perhaps did not main- 
tain a continuous plant cover but the ad- 
jacent uplands did; their cover was prob- 
ably prairie. 
The Illinois River valley south from 
the “Big Bend” at De Pue was established 
in pre-Pleistocene time and has been occu- 
pied by vegetation since Illinoian deglaci- 
ation except for the area from Peoria 
northward, which was glaciated again 
during Tazewell time. The Rock River 
hill prairie site included in this report was 
not covered by ice after Farmdale glacial 
time. The area along the valley of San- 
gamon River near its mouth was last gla- 
ciated during I[Jlinoian time. 
It seems entirely possible that during 
the Tazewell substage, when the ice-front 
was at the Shelbyville moraine, a fairly 
steep climatic gradient prevailed, with cli- 
matic conditions along the valleys in 
western Illinois not very different from 
those of the present. Loess deposition oc- 
curred during the time of advance, maxi- 
mum extent, and recession of the glacial 
ice. The major deposition doubtless took 
place in the autumn and winter, which 
were then, as now, the dry seasons of the 
year. Loess deposits of Kansas contain 
fossil snails that point to a plant cover 
for that area at the time of deposition that 
consisted of shrubs and herbaceous spe- 
ILtiNois NATURAL History SuRVEY BULLETIN 
cies (Leonard 1952) and of a forest bora 
der near the Missouri River (Leonard & 
Frye 1954). As the Kansas deposits are 
Vol. 26, Art. B) 
ee te 
of comparable age to those of Illinois, a_ 
similar plant cover may have existed in ~ 
parts of Illinois. The vegetation in much 
of western Illinois west of the Tazewell 
ice-front may be imagined as that of a 
prairie or grassland (Gleason 1923). The — 
floodplains were devoid of plant cover be- 
cause of the constant shifting of the over- 
loaded streams with their braided chan- 
nels. During the dry autumnal and win- 
ter seasons, the floodplain served as a 
source of silt, which was transported to 
the uplands by the then, as now, prevail-— 
ing northwest winds. 
While separating bunch-grass clumps 
from Reavis Spring to expose individual 
plants (page 381), Vestal and the writer 
observed that vigorous upward growth of 
plants had kept pace with the deposition 
of additional sandy material. From such 
observations it can be concluded that, 
along the bluffs, deposition of loess fa- 
vored rather than discouraged the growth 
of bunch grasses. The inverse also was 
true; a grass cover favored the catching 
and holding of windblown silts (Shimek 
1903). As new areas were exposed by 
deglaciation (for instance in the Putnam 
County hill prairie locations), the prairie 
species, because of proximity and the ra- 
pidity with which many of them could 
become established, moved in and occu- 
pied the bluffs before woody plants could 
do so. Mosses and lichens were not the 
first plants to become established in areas 
of deglaciation. On present-day loess and 
till slopes in Illinois, these cryptogamic 
species are absent from sizable areas 
without vascular plants but are present in 
some interstices where grass plants give 
some protection. A similar situation prob- 
ably prevailed during deglaciation. Prai- 
rie vegetation thus can be assumed to have 
long antedated tree cover on many of the 
bluffs of the larger rivers in Illinois. The 
advantage of priority of occupation is 
probably far more important in succes- 
sion than is commonly realized. 
Although subsequent history has per- 
haps been one of progressive encroachment 
by forest on many of the Illinois blufts, 
especially those of low altitude, several 
