GENERAL FEATURES OF ILLINOIAN DRIFT SHEET. 271 



of which about 3,200 square miles, are in southeastern Indiana, 400 square 

 miles in Kentucky, and 2,800 square miles in Ohio. Its greatest extension 

 beyond the limits of the Wisconsin is 60 miles, in southern Indiana. In 

 the part which extends to the Ohio River the distance of the southern border 

 of the lUinoian from the border of the Wisconsin ranges from 20 to 50 

 miles. By reference to the glacial map (PI. II) the extent of this outlying 

 part of the Illinoian drift may be seen. This map also serves to show the 

 discordance between the Illinoian and Wisconsin drift borders. The Illi- 

 noian border has but one reentrant in the region under discussion, and that 

 a very slight one, while the Wisconsin has two reentrants and three distinct 

 lobes in the area included between the same meridians, and still other lobes 

 farther east. 



The Illinoian drift sheet presents a remarkably flat surface. There 

 are few prominent knolls and no definite morainic ridges except those on 

 the border above described. Much of the surface is so level as to be 

 imperfectly drained. This is especially true in northern Clermont and 

 Brown and adjacent parts of Warren, Clinton, and Highland counties, Ohio. 

 A large area in southeastern Indiana is also poorly drained. 



The very flat surface is found in the part of this region which is under- 

 lain by limestone. The sandstone formations in the eastern part of the 

 region have a more uneven or diversified surface. In the limestone region 

 there appears to have been a gently undulating upland surface similar to 

 that of the "blue grass" region of Kentucky, where only the valleys of 

 the main streams and the lower courses of the tribiitaries are deeply trenched 

 below the uplands. The drift is sufficient usiially to fill the shallow valleys, 

 and in some cases it has so completely filled deep preglacial valleys that 

 their courses are traced with difficulty. Among the sandstone hills it has 

 only partly filled the valleys, though its thickness is nearly as great as in 

 the legion undei'lain by limestone. 



In the headwater portion of the Rocky Fork drainage basin, near 

 Hillsboro, Ohio, there are prominent drift ridges and knolls which lie near 

 the limits of the Wisconsin drift, but which seem to be older than the Wis- 

 consin. They are accordingly discussed in connection with the Illinoian. 

 Near the railway there are several knolls standing either on the valley 

 slopes or in its bottom, which range in height from about 10 feet up to fully 

 100 feet. With the knolls there is found a sharp ridge, 75 feet or more in 



