GEOLOGICAL AND NATURAL HISTORY SURVEY. 15 



It remains to be determined whether it should be made to include 

 all the drift of Southern Illinois and the oldest sheet of drift in dis- 

 tricts to the east from Illinois. Further study is also necessary 

 to determine whether the drift on the border of the Driftless Area 

 in Northwestern Illinois and Southern Wisconsin is so recent as 

 the Illinoian sheet. There may be found in the region referred to 

 a sheet or sheets of drift corresponding in age with one or both of 

 the sheets in Iowa that are earlier than the Illinoian drift sheet. 

 The drift of these regions is thus only provisionally referred to the 

 Illinoian stage. 



The Illinoian sheet has in Southeastern Iowa and Western 

 Illinois a variable thickness, ranging from ten feet or less up to 

 fully 100 feet, with an average of perhaps forty feet. It is scarcely 

 so thick as the older sheet of Iowa and Northern Missouri, whose 

 edge it overlaps in Southeastern Iowa. In Southern Illinois, also, 

 the drift is not so thick as in the old sheets of Iowa and Missouri, 

 being even thinner than in Western Illinois. The same is true of 

 the old drift sheets of Southern Indiana and Southern Ohio. In 

 all these districts the older drift is not so thick as the newer. The 

 Illinoian drift sheet has mainly a plane surface, but near its mar- 

 gin there is a tendency to morainic ridging. The ridging is a sub- 

 dued form, the ridges being one-half to one mile or more in width, 

 with a relief of but tv\^enty-five to fifty feet. The extent of the Illi- 

 noian sheet and the distribution of its ridges are shown on the 

 accompanying map (plate 2). 



Explanation of Map of Illinois. — In this map the extent of 

 the Illinoian drift sheet is represented. Its extension into Mis- 

 souri, near St. Louis, is conjectural, but the extension into Iowa is 

 well established. The several moraines formed in the earlv and late 

 Wisconsin stages are indicated by shading with dots. The extent 

 of Lake Chicago in Northeastern Illinois and Northwestern In- 

 diana is also represented. These features have been added to the 

 map as published by U. S. Geologfica] Survev- The 50 foot 

 contours appear except in the hilly parts of Illinois and in 

 western Indiana where onh^ 100 foot contours are shown. 



THIKD OB PEELOESSIAL INTEKVAIj OF EECESSION OK DEGLACIATION. 



There is between the Illinoian drift sheet and the overlving 

 mantle of loess and associated silts a well-defined soil horizon. 

 There is also leaching of the till to depths seldom less than four 

 feet and often much more. It is, therefore, an interval of con-' 

 siderable importance. Its length, however, appears to be some- 

 what less than that of either of the two earlier intervals. 



