16 FLUORSPAR DEPOSITS OF SOUTHERN ILLINOIS, [bull. 255. 



name the entire ridge. For convenience that portion developed in 

 Pope and Hardin counties will be called by the local name " Karbers 

 Ridge," a name also applied to a small village at the base of its south 

 front. 



Upland plain. — At a lower level and extending southward from 

 Karbers Ridge to the Ohio River is a rolling, broken plain sloping 

 gently from about 600 feet elevation at the north to 500 feet at the 

 soutli. While somewhat irregular in detail, it is in general a mark- 

 edlv level plain. The high Shetlerville Hills rise somewhat above it, 

 but aside from them all the country south of Karbers Ridge in Illi- 

 nois is at or below the level of this plain. In Kentucky there are 

 several hills which rise above it, and in both States the rivers and 

 their tributaries have reduced considerable areas to a lower level. 

 Along the north side of the Ohio such a lower plain is locally devel- 

 oped between Elizabethtown and Cave in Rock. Here a strip ap- 

 proximately parallel to the river and 2 miles wide has been reduced 

 to an altitude of about 440 to 400 feet. North of this strip the coun- 

 try rises very abruptly to 550 or GOO feet, after which the even 

 upland plain is preserved, except for the interruptions due to river 

 valleys, to the foot of Karbers Ridge. North of Golconda there is a 

 distinct bench at about 480 feet above sea, while above that, at 550 to 

 575 feet, is the general upland plain. 



The upland plain is developed upon various sorts of rocks. Lime- 

 stones, sandstones, and shales outcrop at various points within its 

 limits, and all have been cut down to a common level. Locally the 

 individual formations influence the topography, as northeast of 

 Elizabethtown, where the St. Louis limestone underlies the lower 

 plain mentioned, and as in the case of the Shetlerville Hills, twin 

 peaks, conspicuous from every point upon the upland, which are 

 largely composed of the hard sandstone and conglomerate of the 

 Mansfield. The plain is a true peneplain and marks an ancient base- 

 level of erosion. It was formed at a time when the surface of the 

 earth was undisturljed for so long a time that the streams broadened 

 their valleys until the hills between were almost entirely cut away. 

 The time at which this occurred is approximately fixed by the fact 

 that in southern Pope County the plain passes very gradually beneath 

 the gravels and sands which are believed to be of Tertiary age. In 

 southern Illinois also this plain marks the northwestern limit of the 

 Interior Lowland, Avliose age has been fixed as Tertiary at a number 

 of points." 



River valleys. — Into the upland plain the rivers have cut their 

 valleys. For the most part these are rather narrow and sharp walled. 

 The Ohio, the master stream of the area, lias a low-water level of 290 

 feet from Golconda to Elizabethtown. It flows between high rock 



<» Hayes, loc. cit. 



