18 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 496 



LAKE MICHIGAN CORE 969"2A 



Depth . f e° ./^ 



(cm) ^ fV 



5 



9%° 



2 



20 



30 40 



10 15 5 10 15 

 Pollen concentration 

 grains /g dry wt 

 x 1000 



Fig. 7 - Diagram of concentrations of pollen of selected taxa in Lake Michigan 

 core 969-2A. 200 fossil pollen grains were counted from each sample. 



Onthe basis of those cycles, the base of the Waukegan Member has been calcu- 

 lated to be 5540 radiocarbon years B.P. 



DISCUSSION 



The pollen preserved in the sediments of southern Lake Michigan record 

 changes in vegetation that have occurred in the region over the last several thou- 

 sand years. These changes are seen as pollen assemblage zones that are recog- 

 nizable in both the center of, and the eastern side of, the southern basin of the 

 lake. The increase in Ambrosia pollen in response to a historic land clearance 

 is a particularly important vegetation change in that it provides a time marker 

 independent of radiocarbon or paleomagnetic dating. 



