22 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 304 



Faunal Assemblages from Woodfordian Terrace Deposits 



The emphasis in this report is on faunal assemblages from loess deposits, 

 but during our field work we made six collections (fig. 2) from deposits that may be 

 referred to as Woodfordian terraces. The snails from locality 2 were collected from 

 terrace sand and silt; those from locality 6 were recovered from a 6-inch zone of 

 coarse gray silt below Bloomington till; those from locality 7 occurred in sand and 

 silt in deltaic deposits in front of the Arlington (Cropsey) moraine; shells at locality 

 35 were collected from a silty band in a 15-foot roadcut exposure of sand and silt 

 below loess. Only the assemblages from localities 41 and 42 came from typical 

 terrace deposits. An organized study of molluscan faunal assemblages in Pleistocene 

 terraces in Illinois remains to be made. 



The total assemblage from these six collecting stations ranges from six 

 species (localities 6, 7) to 11 species (locality 35). Except for the occurrence of 

 Valvata tricarinata atlocality 41, each of the local faunas has little to distinguish 

 it from Woodfordian assemblages from loess in the near vicinity. In view of the 

 paucity of data from the terraces, no generalities concerning them can, or should, 

 be made at this time. 



SUMMARY 



The recent revision of the classification of the Wisconsinan Stage of the 

 Pleistocene in Illinois (Frye and Willman, 1960) has made possible a systematic 

 evaluation of late Pleistocene molluscan faunas within a firmly established strati- 

 graphic framework. Although Pleistocene mollusks have been known for many years 

 (Baker, 1920), this paper presents for the first time the results of an organized study 

 of fossil mollusks within a discrete segment of Pleistocene time over a significant 

 geographical region of Illinois. 



The molluscan faunal assemblages treated here are grouped under the Altonian, 

 Farmdalian, and Woodfordian Substages of the Wisconsinan Stage; fossiliferous 

 sediments of Twocreekan and Valderan Substages have not yet been recognized in 

 the Illinois Valley region. 



The Altonian Substage is represented by the Roxana silt from the approximate 

 latitude of the mouth of the Sangamon River to Alexander County . The Roxana silt, 

 although not fossiliferous throughout, yields a molluscan fauna of distinctive qual- 

 ities, characteristic of those found in climactic deciduous forests. Only two species, 

 Trlodopsis altonensis and T. hubrlchti, seem to be definitely restricted to Altonian 

 faunas, but the typical assemblage embraces also Alloiona profunda (which occurs 

 outside the Altonian at only one Farmdalian locality), Aniuisplra alternnta, and 

 occasionally A. kochi , Hendersonia occulta, Stenotrema fraternum , Stenotrema 

 hlrsutum , Succinea oualis, and other less frequently occuring species. A total of 

 28 species are known from the Roxana silt at 11 localities. 



Collections from three exposures of Farmdale silts yielded a total of 24 

 species, but because of the widely separated geographic distribution of the localities 

 and the unusual qualities of each, the Farmdalian faunal assemblage is not yet well 

 characterized. In the water-laid deposits in LaSalle and McLean Counties the pre- 

 dominant species are Valvata tricarinata, V. sincera, Amnicola leiihtoni , A. lustrica, 

 and shells of several species of Pisidium . Predominant species in the single southern 

 locality, at the latitude of Gallatin County, are Aniuisplra alternatn, A. kochi, Allo- 

 iona profunda, and Succinea ovalis. This southern fauna, like that from the Altonian 

 of the same region, is characteristic of deep forests. 



