38 ILLINOIS STATE ACADEMY OF SCIENCE 



or degree of stream erosion; second, the character of the 

 rocks immediately beneath the mantle rock; third, the 

 amount and different periods of uplift; fourth, the amount 

 of faulting that occurred during the periods of uplift. 



The greatest contrast in surface features is to be seen in 

 the region of the Pottsville sandstones and conglomerates, 

 compared with the section occupied by the less resistant 

 limestones and shales of the Mississippian period. The 

 Pottsville rocks are found in the northern part of the Ozarks 

 while the weaker Mississippian limestones and shales are 

 found in the southern section of the same highlands. 



The Pottsville being largely sandstones and conglomerates 

 with the cementing material in many cases iron oxide, these 

 rocks are more resistant than many of the limestones and 

 shales of the Mississippian outcrops in the southern part of 

 the highlands. The Pottsville form a continuous strip of 

 land in the southern part of Jackson, Williamson, Saline, 

 and Gallatin counties and extend somewhat less continu- 

 ously along the northern boundary of Union, Johnson, Pope, 

 and Hardin counties. 



Where the resistant Pottsville rocks are exposed, or lie 

 immediately under the mantle rock, the stream made valleys 

 are characteristically narrow and steep sided, but the hill 

 tops have larger summit areas than the less resistant lime- 

 stone areas farther south. Not all of the Pottsville area 

 has these flat topped hills, but this is characteristic of much 

 of this region. 



In the southern part of the Ozarks, however, the land is 

 maturely dissected and flat hill top farms are rare. The 

 surface consists of a series of deep valleys with ridges or ir- 

 regularly shaped hills between the valleys. 



In certain sections, particularly in Hardin county, fault- 

 ing has been responsible for certain surface features. Fault 

 lines often represent places of weakness, where streams can 

 more easily develop their valleys. In Hardin county sev- 

 eral stream courses are thus determined by fault lines. 



There are certain easily dissolved limestones that have 

 caused the characteristic sink hole and underground drain- 



