26 THE ILLINOIS GLACIAL LOBE. 



The greatest width of this area is about 125 miles, and the width nowhere 

 in western or southern Illinois falls much below 75 miles. In southwestern 

 Indiana it occupies a triangular-shaped area, broad at the west and narrow 

 at the east, for the glacial boundary passes northward nearly to the edge of 

 the Wisconsin drift in the south-central portion of that State. 



In the portion of Illinois north from the latitude of Rock Island the 

 Iowan drift occupies a large part of the interval between the glacial bound- 

 ary and the outer moraine of the Wisconsin series. A drift, tentatively 

 referred to the Illinoian, forms the surface sheet in that region in Stephen- 

 son County and parts of Winnebago, Ogle, Whiteside, Carroll, and Jo 

 Daviess counties. A small area of this earlier drift in southwestern Wis- 

 consin is exposed outside the limits of the Iowan, but the exact boundaries 

 of the latter have not been determined. 



If the loess capping be disregarded, nearly two-thirds of the glaciated 

 portion of Illinois has the Illinoian drift as a surface sheet. The remainder 

 of the State is mainly occupied by the AVisconsin drift sheet, the Iowan 

 being exposed only in parts of a few counties. 



Topographic expression. — The greater part of the Illinoian drift has a plane 

 surface, destitute of the swells and ridges which usually occupy the Wis- 

 consin till sheet. There are, however, a few belts of sharply ridged drift 

 found within its area, and the border is ridged throughout much of south- 

 eastern Iowa and western Illinois. The most conspicuous ridging is found 

 in a strip about 20 miles wide leading southwestward through the Kaskaskia 

 Basin from the border of the Wisconsin drift in Shelby County across 

 southeastern Christian, eastern and southern Montgomery, western Fayette, 

 Bond, Clinton, southeastern Madison, eastern St. Clair, and eastern Ran- 

 dolph counties. This strip embraces a series of nearly parallel ridges, which 

 are discussed in some detail farther on as the ridged drift of the Kaskaskia 

 Basin. 



Another ridged belt is found in eastern Sangamon and southwestern 

 Logan counties, and this is discussed below as the Buffalo Hart moraine, 

 the village of Buffalo Hart being situated on it. A less conspicuous belt of 

 ridged drift traverses southern and western Fulton County, touching eastern 

 McDonough County near Bushnell, and apparently having its continuance 

 into Knox and Peoria counties in a chain of mounds and short ridges. It 

 has far less prominence than the two belts just mentioned, its relief being but 



