30 ILLINOIS NATURAL HISTORY SURVEY MANUAL 



Prairie and lake areas. — Outside of the hill regions and the 

 river valleys, Illinois is relatively flat and contains large areas 

 of cultivated land, once prairie, unfavorable to moUuscan life. 

 Although Illinois is primarily a prairie state, a large part of the 

 original prairie a^^as has been so altered by cultivation as to 

 destroy conditions under which snails grow abundantly. Prairie 

 conditions, however, are still found in many localities, some of 

 them isolated. 



In the northeastern area bordering Lake Michigan rise some 

 high moraines, where mollusks may be found. In Lake and some 

 of the adjoining counties may be seen a few bodies of water, 

 but aside from this district there is no area in the state compar- 

 able to Wisconsin and Michigan with respect to multitude of 

 lakes, large and small. The dune-bordered shores of Lake Mich- 

 igan afford but poor habitats for land mollusks. 



Mollusks in Geological History 



Mollusks occur in the oldest stratified rocks, the Cambrian, 

 laid down over 600 million years ago. However, these are all 

 marine animals, snails and clams, a few of which are known 

 from the rocks of Illinois. The first known land snails are from 

 the Carboniferous rocks of the Coal Period, about 300 million 

 years ago. Most of these have been found in fossil tree stumps 

 in Nova Scotia. Several land snails are known from the coal 

 fields of Pennsylvania. In the great Chalk Period, the Cre- 

 taceous, land snails became common, and in the period known 

 as the Tertiary, about 60 million years ago, they became abun- 

 dant. Their number has increased to the present time. No ancient 

 land snails are at present known from Illinois, but it is very 

 likely that such animals lived here, as in Nova Scotia and 

 Pennsylvania, among the luxuriant vegetation of the Coal 

 Period. 



Glacial periods. — Very late in geological time a glacial or 

 ice period developed which sent great ice fields into Illinois and 

 other parts of the United States. With the exception of a small 

 area in the northwestern part, mostly in Jo Daviess County, 

 and a larger area in the extreme southern part of the state, Illi- 

 nois was completely covered with an ice sheet. The ice came 

 into and retreated from Illinois at least three times, the first 

 invasion, the Kansan, as well as the second, the Illinoian, 

 reaching nearly to the southern boundary of the state, stopping 



