200 ILLINOIS STATE ACADEMY OF SCIENCE 



further development. In 1915, Rich 1 made a careful 

 study of the Birds Quadrangle in Crawford County, and 

 noted the allignment of the Pennsylvanian sand produc- 

 tion at right angles approximately to the axis of the anti- 

 cline. He concluded that the Pennsylvania!! beds were 

 essentially flat-lying over this area, and that oil accu- 

 mulation in the Robinson sand seemed to be conditioned 

 on its thick and massive development. Differences in 

 elevation of the sands due to arching and folding are less 

 in amount than differences due to irregularities in the 

 sand lenses themselves. 



Later work by Mylius 2 in Clarke County disclosed cer- 

 tain features in the northern part of the field, to account 

 for which he hypothecates cross folding. As some of the 

 production includes pre-Pennsylvanian rocks, the prob- 

 lem is modified somewhat, but the fact is noted that 

 something other than the LaSalle anticline is involved in 

 the oil occurrences in this part of the field. Recent field 

 work, as yet incomplete, in the north one-half of Craw- 

 ford County is adding its quota to the information rel- 

 ative to our south-eastern oil fields and furnishes some 

 conspicuous contrasts between Pennsylvanian and pre- 

 Pennsylvanian production. Here the upper sands (Rob^ 

 inson) are elongated in a direction approximately N. 52° 

 E, whereas upper Mississippian production (Chester), 

 developed here in only a small way, follows the direction 

 of the anticline. As noted by Rich in the Birds Quad- 

 rangle, differences in elevation of the top of the Robinson 

 sand are influenced less by folding and warping than by 

 lensing. Some of these areas are irregular in shape with 

 no conspicuous or symmetrical elongation. Others are 

 several miles in length, and these longer dimensions are 

 in the northeast-southwest direction. Another note- 

 worthy feature is the occurrence of all of these elongated 

 areas on the northeast side of the axis of the anticline. 

 In no instance have they been reported or observed on 

 the west flank of the fold. The most prominent develop- 

 ment of this nature occurs between Birds and Annapolis, 

 a distance of about twenty miles. 



1 Rich, J. L,., Oil and Gas in the Birds Quadrangle, 111. State Geol. Sur- 

 vey, Bull. 33, pp. 105-145. 



2 Mylius, L. A., Extract from Bulletin 44, 111. State Geol. Survey. 



