40 THE ILLINOIS GLACIAL LOBE. 



decidedly morainic expression to the drift surface. Occasional knolls and 

 low ridges of drift were found in northeastern Gibson and northern Pike 

 counties, Indiana, lying usually within 5 miles of the glacial boundary. 

 No knolls or ridges of drift were noted in the district north of East White 

 River, nor have any been observed far back from the glacial boundary in 

 southwestern Indiana and southern Illinois. 



Reviewing the above statements, it appears that the border is only in 

 places marked by a definite ridge, and that there the ridge has mild expres- 

 sion and slight dimensions compared with the expression and dimensions of 

 the bulky moraines formed at the Wisconsin stage of glaciation. The 

 expression is also much milder than that of ridges formed at some distance 

 back from the drift border in southwestern Illinois, which pertain to the 

 Illinoian sheet, descriptions of which are given below. 



STRUCTURE OF THE DRIFT BORDER. 



The drift border, as here discussed, includes a belt several miles in 

 width, embracing a sufficient amount of territory to afford a fair index of 

 the variations which are displayed in the immediate vicinity of the border. 



The discussion begins in Lee County, Iowa, which is the writer's native 

 county, since the sections of wells and also natural exposures have been 

 studied more thoroughly there than at any other part of the drift border. 

 This county is situated in the extreme southeast corner of Iowa. (See fig. 4). 

 During the drought of 1894 and 1 895 a large number of new wells were sunk, 

 and the writer had opportunity to make many observations concerning the 

 character of the drift penetrated by them. The Illinoian drift sheet is found 

 to be generally but 10 to 30 feet in thickness, though on the ridge which 

 forms its western limit the thickness is increased to 50 feet or more. In 

 several of the wells which were observed during excavation the Illinoian 

 drift is composed of a brownish, pebbly clay, which has been so thor&ugkly 

 leached that no response with acid could be obtained, even where the thick- 

 ness is 20 feet. But in some of the wells this sheet contains a very cal- 

 careous till which has been leached only to a depth of 6 or 8 feet. This 

 variability in the amount of leaching is thought to be due to difference in 

 the derivation of the material. That which is leached from top to bottom 

 is probably made up in large part of the surface portion of the older sheet 

 of drift which is here overridden. That which is a typical calcareous till 



