THE CHESTER SERIES IN ILLINOIS 399 



Menard limestone. — ^The Menard limestone was originally 

 named from the exposures in the Mississippi River bluffs at Menard, 

 IlHnois.^ The formation is a persistent one in the Chester section 

 of the Mississippi River counties of Illinois, from the point where 

 the formation is first exposed from beneath the overlapping Penn- 

 sylvanian formations in Randolph County to where it passes 

 beneath the surface in the Mississippi River bluffs in Jackson 

 County. In the Mississippi River counties, where it was first 

 described, the Menard was found to rest upon the Okaw Kme- 

 stone, the two limestones being rather sharply differentiated by 

 both lithologic and faunal characters. It is now known that two 

 important sandstone formations, the Tar Springs and the Walters- 

 burg, are intercalated between these two limestone horizons in the 

 Chester section of the southern counties, and also the Vienna 

 limestone, unless it can be shown that this formation is represented 

 by the black-shale bed that is at least locally present in Randolph 

 County between tjie top of the Okaw limestone and the base of 

 the more typical portion of the Menard. 



In its typical expression the Menard is evenly bedded in layers 

 about one foot, more or less, in thickness. The individual beds 

 are separated by thin shaly seams or by beds of shale up to several 

 feet thick, the surfaces of the limestone layers being distinctly 

 hummocky. The Hmestone itself is commonly compact in texture, 

 ahnost lithographic in places, hard and rather tough, breaking 

 with a splintery or conchoidal fracture. The color of the lime- 

 stone is commonly light gray, and the weathered surfaces are smooth 

 in most places. These characteristics are quite in contrast with 

 the crystalline beds of the Okaw limestone, with its more roughly 

 weathered surfaces. An occasional bed of crystalline limestone is 

 met with in the Menard, which might be mistaken for the Okaw if 

 it were found alone. 



Where the Menard limestone is exposed in the Chester belt 

 across the southern counties, it can be easily traced across Union, 

 Johnson, Pope, and Hardin counties, except where it is interrupted 

 by faulting. Throughout this belt the limestone exhibits essen- 

 tially the same lithologic characters as in Randolph County, 



^ Weller, Trans. III. Acad. Set., Vol. VI (1914), p. 128; also ///. State Geol. Sim., 

 Monog. I (1914), p. 28. 



