BAKER: FIELDBOOK OF ILLINOIS LAND SNAILS 



31 



on the north side of the hills of the Ozarkian Uplift. The third 

 invasion reached only the eastern and northeastern parts of the 

 state as far south as Shelbyville in Shelby County. 



Limestone bluff in Giant City State Park, Jackson County. Snails 

 are found in the woods above the bluff and in the rock crevices. 



These huge ice fields brought down great quantities of 

 clay and rock, which were deposited at the edge of the ice to 

 form moraines, the ridges which are now a marked feature of the 

 landscape in various parts of the state, as near Champaign, 

 Shelbyville, Bloomington, Paxton, Pontiac, Aurora, Algonquin 

 and Summit. 



Interglacial intervals. — The geological time of the huge ice 

 fields, known as the Pleistocene or Glacial Period, lasted about 

 one million years, divided among the several ice invasions. 

 The periods between these invasions, called interglacial inter- 

 vals, were probably of long duration, thousands of years, and the 

 climate at its most genial time is believed to have been warmer 



