402 ILLINOIS STATE ACADEMY OF SCIENCE 



The limestone is gray-wliite, coarse grained, and moder- 

 ately hard, and is unconformable with the beds above and 

 below. 



THE ORDOVICIAN SYSTEM 



Maquoketa shale: 



The Maquoketa is a dens'e, dark gray shale. Its thick- 

 ness, as indicated from well logs, averages about 85 feet 

 on the top of the anticline. Well down on the flanks of 

 the anticline its thickness is as great as 145 feet. 



Fernvale limestone or '^Trenton Cap" : 



Figures on the thickness of the Fernvale are very dif- 

 ficult to obtain, primarily because it is followed below by 

 a limestone, with which it is commonly included by the 

 drillers. This is not surprising because locally the top of 

 the Kimmswick ''tightens up" so as to be difficult to dis- 

 tinguish from the Fernvale without careful examination. 

 However, from the best available information, the aver- 

 age thickness of the Fernvale may be considered about 

 12 feet. The minimum thickness noted is 2 feet, 

 in the log of a well drilled approximately on the axis 

 of the anticline, and the maximum thickness of 29 feet, 

 somewhat off on the east side of the anticline. The Fern- 

 vale is a dense, fine grained, hard, brittle, thin bedded, 

 white limestone. 



Kimmsivick limestone {"Trenton") : 



The Kimmswick is a coarse grained, moderately por- 

 ous, coarsely crystalline, white limestone, and tests above 

 95 per cent calcium carbonate. When bailed out from a 

 well it resembles very closely a true sand, and is so called 

 by the drillers. Particularly is this so when the lime- 

 stone does not contain oil, and is therefore pure white. 

 The coarsely crystalline character and porosity make 

 the Kimmswick a good oil reservoir, though locally it 

 is rather "tight", particularly in its upper portion, and 

 well down on the sides of the anticline. The top of the 

 Kimmswick is irregular and unconformable with the 

 formation above. 



