226 ILLINOIS STATE ACADEMY OF SCIENCE 



mation, which completely wedges out as the anticline rises 

 to the north. In Lawrence County the lower Mississippian 

 is all limestone; in central Clark County the basal member 

 is 250 feet of sandy shale, overlain by 200 feet of sandy 

 lime; in southern Coles County the Lower Mississippian 

 has less lime and more shaly sandstone; in northern Coles 

 County the basal 500 feet is all shaly sandstone without 

 lime. East and west off the structure, the limestone mem- 

 bers appear over this basal sandy phase. This increasing 

 sandy character of the beds as one goes north is undoubt- 

 edly due to the closer approach of the "source of supply," 

 which lay north or northeast during Mississippian times. 



CHESTER TIME 



The Chester lies unconformably on the Lower Missis- 

 sippian. 



In Lawrence County the Chester is thickest and it be- 

 comes progressively thinner to the north along the anti- 

 cline until it disappears in southern Clark County; and 

 from any given point on the crest of the fold the Chester 

 thickens both to the east and to the west. Since the Chester 

 includes beds of Lower Chester age even where it is thin, 

 and since the same sand horizons as are found where it is 

 thicker seem to be present in the Chester where it is com- 

 paratively thin, it may be inferred that the northward 

 thinning of the Chester and especially its thinning over the 

 crest of the fold are due to erosion subsequent to the depo- 

 sition of a more or less complete section of Chester over the 

 whole area. In agreement with such an inference, the 

 Chester contact with the overlying Pottsville shows a very 

 marked unconformity which indicates that the erosion in- 

 terval in places must have been great. Of course, a part of 

 the northward thinning may be credited to the fact that 

 the Chester shoreline possibly lay to the north, but it can 

 not all be explained in this way. 



The fact, previously mentioned, that Lower Chester beds 

 persist into the areas where the Chester is thin, shows that 

 no decided diastrophic movements took place in this area 

 until after Lower Chester times. However, the repeated 

 alternation of sandstones, shales, and limestones through- 



