22 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 487 



most ancient (geologically speaking) of the terrestrial gastropods. Typical ex- 

 amples of Vertigo have been recovered from continental sediments of early 

 Cretaceous age. 



Vertigo modesta (Say) 1824 



Pupa modesta Say 1824, Long's Exped., Appendix, p. 259, pi. 15, fig. 5 

 Vertigo modesta (Say), Pilsbry 1948, Land Moll. N. America, v. 2, 

 pt. 2, p. 982, figs. 528, 531. 



Vertigo modesta is a species of wet situations, but it is nevertheless a 

 strictly terrestrial pulmonate gastropod. Empty shells of pupillids can float for 

 long periods of time and over long distances; therefore an occasional shell in a 

 water-laid deposit is not at all remarkable. 



In the present study, V. modesta is represented by a single shell recov- 

 ered from the uppermost zone of locality 135-2. This occurrence is not consid- 

 ered significant in the interpretation of the total faunal assemblages. 



ECOLOGICAL IMPLICATIONS OF THE BOGS AND THEIR MOLLUSCAN FAUNAS 



Depositional Ecology of the Bog Sediments 



It is evident from the local topography (figs. 1, 2) that each of the two 

 bogs under study originated in an undrained depression in morainal topography. 

 To what extent, if any, isolated glacial ice masses filled these depressions is 

 not clear. It is also unclear to what extent the original lakes in these depressions 

 formed from meltwater in situ and to what extent by runoff after precipitation on 

 the surrounding slopes, nor does it seem very important. Possibly the temper- 

 ature of meltwater from a slowly dissipating ice mass might have had some inhib- 

 iting effect upon the development of plants and animals in the associated lake, 

 but at the latitude of northeastern Illinois this does not seem likely, especially 

 under the climatic regimen that was producing ablation of the last Woodfordian 

 glacier in the area at the time. A sizable flora and microfauna have developed, 

 for example, in glacier-fed lakes in Antarctica, where climatic extremes are 

 certainly greater than those that could have been expected in Illinois under then 

 prevailing conditions. 



Whatever may have been the original source of the water in these lakes, 

 it seems obvious that they have been maintained by runoff from the surrounding 

 slopes since the lakes began accumulating sediments some 10,000 or more 

 years ago. Furthermore, the lakes attained in former times a level distinctly 

 higher than the present level. Evidence for this is especially clear at the Strawn 

 NE locality, where a plain some 200 yards wide, north of the main body of the 

 lake, is judged to be a wave-cut terrace about 10 feet higher than the present 

 level of the water (figs. 1,2). 



The deposits in the former lakes that provide the samples from which the 

 molluscan faunas were obtained certainly owe their origin to materials washed 

 from surrounding slopes; although this conclusion needs no defense in a closed 

 basin, the best positive evidence here is that the less than 2 micron fraction of 

 clay minerals is indistinguishable from that obtained from the tills of the area 

 (H. D. Glass, 1972, personal communication) . 



