658 



F. H. KAY OIL FIELDS OF ILLINOIS 



clip from the main fields to the basin is much more pronounced, and the 

 same is true for the beds at the southern and the southwestern rim of the 

 basin. East of the main oil fields of Crawford and Lawrence counties the 

 strata rise gently into Indiana. At the southern end of the State diastro- 

 phism has been exception all}' active and the resulting structural condi- 

 tions are unusual for Illinois. Strong folds and faults are numerous and 

 in places igneous intrusion has occurred. 



The main irregularit}^ in the general structure is the La Salle anti- 

 cline, which extends northwest-southeast through the eastern part of tlie 

 State. Not only is this anticline asymmetrical, but its axis plunges 

 toward the southeast. 



Figure 2. — Unconformity 'between the Chester and Pottsville Formations 

 Two miles south of Pomona, Illinois. (From Bulletin 35, Illinois Geological Survey) 



The Duquoin monocline is the most conspicuous modification of the 

 structure in the western part of the State. The fold is clearly marked 

 from the Sandoval oil field southwest to the town of Duquoin, where it 

 turns more westward and is traceable into Jackson County. West of the 

 axis the beds lie ahnost flat, with slight dips north and northwest, whereas 

 for some distance east of the axis the eastward dip is 300 feet per mile. 



PERIODIC OF DJASTL'OPIIISM 



Several distinct periods of fohiing have occurred in dilferent parts of 

 the State, but it has usually affected the same lines of weakness. At I^a 

 Salle the evidence is clear that a period of diastrophic action followed 



