CHRONOLOGY AND MOLLUSCAN PALEONTOLOGY 



OF TWO POST-WOODFORDIAN BOGS 



IN NORTHEASTERN ILLINOIS 



A. Byron Leonard 



ABSTRACT 



Hand -auger borings intwopost-Woodfordian basic 

 bogs, one northeast of Strawn in Livingston County and 

 one west- southwest of Batavia, Kane County, Illinois, 

 provided the materials for collections of fossil mollusks 

 from a total of 27 levels. Each collection consisted of 

 approximately 1,000 shells; faunal assemblages varied 

 from 10 to 19 species. In all, 31 species, grouped in 

 18 genera, were recovered from fine-grained water-laid 

 sediments; sediments and molluscan assemblages from 

 the two bogs were remarkably similar. Each of the bogs — 

 and their remnant lakes — occupies a basin in morainal 

 topography; the lakes are maintained at present by runoff 

 from surrounding slopes and by groundwater inflow. 

 Radiocarbon analyses of carbonaceous materials in the 

 samples range from about 9,000 years B.P. several feet 

 above the bottom of the sediments to about 2,000 years 

 B.P. at the base of the sphagnum matte that essentially 

 terminated the biological activity in the bog lakes. It is 

 estimated that the deposition in the basins began more 

 than 10,000 radiocarbon years B.P. 



Six species, Amnioola gelida , A. leightoni , 

 Gyraulus altissimus , Helisoma antrosa , Physa gyrina 

 hildrethiana , and Valvata tvioavinata , occur throughout 

 the sediments in both of the bogs; of these, three species 

 are branchiates and all are aquatic. Except for four spe- 

 cies of terrestrial habit (three of these represented by 

 single specimens), the entire fauna in the two bogs is 

 aquatic in habitat requirements. Specific data concerning 

 the habitat requirements of these freshwater mollusks are 

 lacking, but a comparison of the fossils with their living 

 counterparts, where available, indicates that the mollus- 

 can faunal assemblages in the two bog lakes lived in 

 clear, cool, moderately eutropic waters, basic in reaction, 

 sufficiently supplied with dissolved carbonates to sup- 

 port strong growth of molluscan shells, and characterized 

 by fluctuating levels. 



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