16 Illinois Natural History Survey Circular 49 



Varied elements of the eastern North American segment of the 

 temperate deciduous forest have evolved into a great variety of dif- 

 ferent species. Examples of these are the trilliums and violets. The 

 best-known examples of species multiplication are found in the in- 

 sects. In genus after genus of these little creatures, species isolated 

 long ago in the eastern deciduous forest have evolved into groups 

 of 15 to 20 species, and in some instances into flocks of 300 to 400 

 species. 



What may have been a relatively sparse assemblage of species 

 20 million years ago has evolved into a large and blossoming fauna 

 and flora. 



SHIFTING CLIMATES 



There is good evidence that, as the Cenozoic Era proceeded, the 

 climates became progressively cooler. Under these conditions the 

 temperate deciduous forest gradually supplanted the more tropical 

 forests in the area south of Illinois, and the northern coniferous 

 forests may have extended farther south than they do now. Two or 

 three million years ago the Dunesland area may have been covered by 

 this coniferous forest. The new techniques for analyzing fossil pollen 

 may some day give us evidence about this. 



The most spectacular changes in living conditions here in Illinois 

 were produced by the glaciers of the Pleistocene Epoch or Ice Age. 

 In the Dunesland, evidence for these great glaciers is present in the 

 form of pebbles scattered along the beach and mixed with the sand 

 on the ridges. Very few of these pebbles are exactly alike. They are 

 not composed of the limestone and sandstone which occur at the sur- 

 face in Illinois; they are hard, granite-like (or granitic) pebbles. 

 There are no Illinois outcrops from which these pebbles could have 

 come. The question is, how did they get here? The most obvious 

 answer is that they were carried to their present positions from the 

 neighboring outcrops of granitic rocks, the closest of which are in 

 central and northern Wisconsin and Michigan. More of these granitic 

 outcrops occur even farther to the north and east. When the glaciers 

 of the Ice Age overrode the northern countryside, they loosened, 

 picked up, carried along with them, and broke into fragments these 

 granitic rocks. When the glaciers melted away, they left the granitic 

 fragments from which the Dunesland pebbles were formed. 



The Ice Age glaciers must have been among the most awesome 

 sights we can imagine. Starting from centers in Canada, immense 

 glaciers formed and moved by their own weight over tremendous 

 areas of North America, fig. 11. Four major ice sheets descended 



