BAIN.] PHYSIOGRAPHY. 15 



counties by Karbers Ridge, and its northern limit is defined by the 

 South Fork of Saline River and certain tributaries of the Big Muddy. 

 The Interior Lowland has in this area a general altitude of 500 feet. 

 It merges with the Gulf Plains along a sinuous line marking the over- 

 lap of Tertiary deposits upon an erosion plain beveling older rocks. 

 It abuts sharply against the Ozark Plateau, which rises abruptly 

 200 to 250 feet above it, and which also overlooks the Prairie Plains 

 to the north, which lies here at a general altitude of about 450 feet. 

 Below the Interior Lowland and the Gulf Plains the Ohio and other 

 rivers of the district have cut important channels to a general con- 

 trolling base of 300 feet. 



TOPOGRAPHY. 



Karbers Ridge. — Crossing the northern portion of Pope and 

 Hardin counties is a belt of high land presenting an abrupt escarp- 

 ment face to the south. It is limited on the south approximately 

 by the southern boundary of the Mansfield sandstone (see PL I). 

 The escarpment rises 150 to 200 feet above the rolling plain at its 

 base. The ridge itself is narrow and in Pope County reaches out 

 in long, finger-like minor ridges between the southward-flowing 

 streams. The main ridge rises to TOO or 750 feet above sea level, 

 and is the most notable topographic feature of southern Illinois. 

 As already indicated, it extends westward to the Mississippi, beyond 

 which it is represented in the Ozark Plateau. To the east it becomes 

 less and less distinct until lost in the uplands of western Kentucky. 



In Hardin County it has a general east-west course and is excel- 

 lently displayed. In T. 3 N., R. 8 E., and T. 3 N., R. 9 E., it is very 

 sharply defined, the southward- fronting escarpment forming a great 

 frowning wall. In Pope County the trend of the escarpment be- 

 comes more southwesterly. About 5 miles northwest of Golconda it is 

 crossed by the road to Raum, the ridge here forming a narrow pro- 

 jecting point of high land between the branches of Lusk and Little 

 Grand Pierre creeks. It rises here to nearly 800 " feet above sea, 

 approximately 230 feet above the general upland at its base and 

 500 feet above the river at Golconda. Farther west it has not been 

 especially studied. 



On the north side the ridge in Gallatin County presents an 

 even more striking front. From its base stretches the great plain 

 of central Illinois, underlain by the soft beds of the Coal Measures. 

 Farther west the northern boundar}^ is not so well defined. 



Various names have been applied to this ridge at different points. 

 There has been as yet no adequate physiographic study of the region 

 as a whole, and for the present no attempt need be made to define and 



a AU elevations are based on barometer determinations and are approximate only. 



