394 



Illinois Natural History Survey Bulletin 



Vol. 26, Art. 5 



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 Ri\er valley in western Illi- 

 nois has had a \egetational 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, AVhiteside County, and 

 Cordova, Rock Island Count\ , which was 

 covered during 'ia/cwell glacial time 

 (Shaffer 1'I54), 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 \alle\ perhaps did not main- 

 tain a continuous plant cover but tlie 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-PIeistocene time and has been occu- 

 pied by \egctation 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 Illinoian 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- 



cies (Leonard 19521 and of a forest bor- 

 der near the Missouri River (Leonard 5: 

 Fr\e l')54). As the Kansas deposits are 

 uf 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 de\oid of plant cover be- 

 cause of the constant shifting of the over- 

 loaded streams with their braided chan- 

 nels. During the dn,- autumnal and win- 

 ter seasons, the floodplain served as ;i 

 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 sand\- 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 b\ 

 dcglaciation ( 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 bluffs, 

 especially those of low altitude, several 



