BAKER: FIELDBOOK OF ILLINOIS LAND SNAILS 29 



northeast, Lake Michigan washes the shores for many miles. 

 Within the state' area are several large river valleys. The Illi- 

 nois, which is the largest river in the United States lying wholly 

 within the borders ot a single state, the Rock, Kaskaskia, Big 

 Muddy, Little Wabash, Embarrass, Kankakee, Fox and Sanga- 

 mon rivers afford many attractive valleys, usually well wooded 

 and providing favorable habitats for snails. Tributaries of these 

 streams torm a great network of waterways throughout the 

 length and breadth of the state. Scarcely another state of the 

 Union has so many stream valleys, which means that Illinois 

 presents unusually good hunting grounds for the conchologist. 



Plain and valleys. — Much of Illinois is an undulating plain 

 which in a few places has been rather deeply dissected by streams 

 and rivers. In the central and southern parts of the state some 

 of the river banks are towering cliffs which with their castellated 

 crests form picturesque features of the landscape. The bluffs of 

 the Mississippi attain a height 360 feet above the level of the 

 river. The banks of the Ohio rise in places as m.uch as 100 or 200 

 feet above the water. In the lower Illinois valley, in Calhoun 

 and Jersey counties, some of the cliffs reach an elevation of 

 approximately 300 feet above the river. The banks of the Rock 

 River, 'especially those near Oregon, are famed locally for their 

 height. Terrain of this rugged type, although not representa- 

 tive of Illinois as a whole, furnishes favorable habitats for cer- 

 tain species of snails. 



Hill regions. — South of Harrisburg and Carbondale is an 

 eastward extension of the mountains of Missouri, known as the 

 Ozarkian Uplift. This area, which in places attains an elevation 

 of 1,100 or more feet above sea level, harbors a greater variety 

 of land Mollusca than any other section of Illinois. The dis- 

 section of this uplift by erosion has produced many hills, some 

 of them precipitous, which rise 350 to 650 feet above adjacent 

 valleys. 



In Jo Daviess County is another hill region which extends 

 into Wisconsin and is sometimes called locally "the little Switzer- 

 land of America." Although this region lies at approximately 

 the same distance above sea level as the Ozarkian Uplift, its 

 hills are less precipitous and do not rise to such heights above 

 adjacent valleys. 



Neither the Jo Daviess County nor the southern Illinois 

 hill regions felt the influence of the great glaciers which in other 

 places planed the hills and filled the valleys with debris. 



