Correlation of Data with Phenomena in Adjacent Areas. 505 
The results of Mr. Leverett’s work in adjacent areas of 
Illinois are of greatest interest in this connection inasmuch as 
they cover in part the area under consideration. He has dif¬ 
ferentiated in this state a broad outer belt of much oxidized 
till with a surface of old soil which is itself covered over large 
areas by a loess-like silt. This bears on its surface numerous 
belts of stratified drift arranged over portions of the area in 
lines concentric with the drift margin, at others in lines parallel 
with the glacial movement and indicative of the drainage lines 
along the retreating ice margin. This drift area in southern and 
western Illinois is from 60 to 100 miles wide and has hitherto 
been considered by him to be a simple deposit, the equivalent 
of what in Iowa Mr. G-ee calls the lower till. According to our 
combined observations on the drift phenomena in the valley of 
Pecatonica the bowlder trains both of the westward and of the 
old southward distributions belong to this oldest till. The old 
silt deposits which are very strongly developed within the valley 
of the Pecatonica overlie both marginal drift and the adjacent area 
of the southward fan, and disappear only on the kame-marked 
borders of the drift deposits here correlated with the south- 
westward bowlder trains. 
The area of comparatively recent drift deposit with its modi¬ 
fied drumlin forms and its semi-morainic marginal deposits 
here described and mapped in connection with the southwest- 
ward bowlder tr.ain, Mr. Leverett traces down the Pock River 
valley to the Mississippi aud considers it an equivalent of Mr. 
Gee’s upper till of Iowa. The extensive silt deposits overlying 
the surface of the outer belt gradually thicken as they approach 
the area occupied by this belt, but disappear upon its margin, 
very little silt being found on the surface of this later drift. 
Its deposition is, therefore, considered to be coincident with 
the melting of the ice sheet over the adjacent area. 
Overlying this drift sheet on the southeast margins of the 
Rock River valley Mr. Leverett traces the margin of a later 
drift bordered by a massive belt of morainic material which he 
has designated as the Shelbyville moraine. This covers an area 
of from fifty to one hundred miles wide from the center of the 
state eastward to the Kettle moraine. Its northern margin dis- 
