10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 461 



Toulon Member, and one from the Mulberry Grove Member of the Glasford Forma- 

 tion. Two collections are from undifferentiated deposits of the Glasford Forma- 

 tion, and one is from the upper part of the Loveland Silt. In all cases the fossils 

 occurred in slack-water silts or in silt and fine sand. With the exception of the 

 collection from the Loveland Silt at the Kintown Hollow Section (fig. 3) (and pos- 

 sibly also from the Birds Corners Section [fig. 3], where the stratigraphic rela- 

 tions are not fully clear), all of these faunas were collected from deposits occur- 

 ring between tills or between tills and coarse outwash. 



In the stratigraphic sections included with this report, the Toulon Member 

 of the Glasford Formation is described in the Hinkle Branch Section, and the un- 

 differentiated Glasford Formation is described in the Harmattan Strip Mine Section 

 No. 3. Of the remaining localities, the Roby Silt Member of the Glasford Forma- 

 tion Is described in the Allis-Chalmers, Roby, and Sherman Sections by Johnson 

 (1964), and the Mulberry Grove Member of the Glasford Formation is described in 

 the Mulberry Grove Section by Jacobs and Lineback (1969). 



FOSSIL MOLLUSCAN FAUNAS 



The molluscan faunas reported here were collected for the most part by 

 bulk sampling, although specimens exposed on the surface were collected indi- 

 vidually. Bulk samples generally consisted of fifty to one hundred pounds or 

 more of matrix containing shells. Samples were dried and then washed over 

 screens of appropriate mesh, a procedure which resulted in the recovery of most 

 of the shells together with roots, coarse matrix, and other detritus. The concen- 

 trate was then dried, and the shells were sorted from the mixture, after which 

 they were boiled briefly in water containing a non- foaming detergent. This pro- 

 cedure cleaned the shells, including their apertures, which are especially impor- 

 tant in pupillid shells, which cannot be identified without reference to the apertural 

 denticles. Following the cleaning process, the shells were finally dried and sorted 

 to species. 



Such collecting techniques produce maximum numbers of individuals, but 

 even then the collection may not accurately represent the faunal complex as it 

 was at the time the sediments were being deposited. Several factors tend to skew 

 the results: namely, the size of the sample taken; the nature and areal extent of 

 the exposure of fossiliferous sediments; the facie s exposed and available for study; 

 the lack of useful exposures in some critical areas; relative fragility, shape, and 

 size of shells of different species of mollusks, each of which importantly influ- 

 ences their transport, deposition, and preservation; and others. Faunal collections 

 made within a few feet of each other have been shown to vary considerably. 



In spite of these variables, however, a significant consistency among assem- 

 blages of shells exists where many collections are made at the same, or nearly 

 the same, stratigraphic level. 



It is remarkable, considering the relatively large number of bulk samples 

 studied, that virtually no vertebrate remains were found, although the methods 

 used are equally applicable to the collection of the teeth and bones of small ver- 

 tebrates. Except for a few isolated teeth of small rodents, no vertebrate fossils 

 of any kind have been encountered in these studies. The reasons for their absence 

 remain obscure, since small vertebrate animals must have coexisted with the mol- 

 luscan faunas represented by the shells herein reported. 



