POST-WOODFORDIAN MOLLUSKS 23 



In each case, the drainage area supplying runoff to the bog lakes is 

 small; Strawn NE (fig. 1) has an effective drainage area (bounded by the 750- 

 foot contour) of less than a square mile. At Batavia W (fig. 2) the drainage area 

 is somewhat larger (bounded approximately by the 720-foot contour) and en- 

 compasses well over a square mile. 



The rate of sedimentation into the bog lakes has been slow. Assuming a 

 conservative figure of 10,000 radiocarbon years B.P. as the age of the deposits 

 and 200 inches as the depth of the sediments, the average rate of accumulation 

 has been 0.02 inches per year, or 2 . inches per century. In spite of the small 

 size of the drainage areas of the two bogs, it appears that: (1) rainfall during 

 the post-Woodfordian interval must have been minimal, and/or (2) the slopes 

 must have been well protected from erosion by a dense vegetative cover. 



At the Strawn NE bog lake, the sediments are extremely fine grained, 

 and pebbles are rare except near the base of the lake deposits. The borings 

 might well have missed any gravel trains from entering drainage, but another 

 observation is more difficult to dispose of. For many years, the marl deposits 

 at Strawn NE were removed by a jury-rigged dragline that stretched for a 

 long way across the lake and was situated near the entrance into the lake of a 

 drainageway from the east (fig. 1) . Where the dragline scoop was emptied into 

 trucks, a large amount of marl collected, and since abandonment of the har- 

 vesting operation several years ago, this "spoil pile" has been subject to wind 

 and rain. Nevertheless, no pebbles were to be found, although the accumulation 

 must represent a rather large sample area. The conclusion seems inescapable 

 either that runoff into the bog lake was not competent to move rock particles 

 larger than silt and very fine sand, or that larger particles were not available 

 because of the protection of covering vegetation on the surrounding slopes. 

 Heavy vegetative cover should result in populations of terrestrial mollusks, but 

 there is almost no evidence of these animals, from the borings, from the large 

 numbers of fossil shells exposed by recent wave action, or from the hundreds of 

 shells exposed on the spoil pile previously referred to. 



The sediments contain fossil plants (diatoms) in the basal sediments; 

 in the upper half of the deposits, plant remains become very conspicuous, but 

 are decomposed beyond the possibility of identification. A few fossil seeds 

 were recovered, most of them from sedges {Cyperus sp.). Pollen was not inves- 

 tigated; but since the bogs are alkaline, pollen may be poorly preserved. 



Each of the two bog lakes terminated in a matte of sphagnum peat, not 

 more than 12 to 18 inches thick at Strawn NE but 4 to 5 feet thick at Batavia 

 W. The base of this peat is approximately 2,000 radiocarbon years old 

 (ISGS-134). 



Ecological Implications of the Molluscan Faunas 



The molluscan faunal assemblages recovered in the two bog deposits 

 are predominantly aquatic in habitat requirements; of the six taxa that occur at 

 every level in both bogs (fig. 3), all are aquatic and three are branchiate 

 gastropods. Ten species occur in more than 20 of the 31 faunal assemblages; 

 of these, five are obligate aquatic animals {Amnicola gelida, A. leightoni, 

 Valvata trioarinata, Pisidiwn nitidum, and P. eompressum) using gills for gas- 

 eous exchange, although they may be able to exist for brief periods of time 

 without open water. Terrestrial gastropods are very rare in these collections; 



