I 28 ILLINOIS ACADEMY OF SCIENCE 



7. Menard Formation. 



General characters. The Menard hmestone is a con- 

 spicuous formation, well exposed in the middle portion of 

 the blufifs at Chester. One of the best exhibitions of the 

 formation is to be seen immediately southeast of the hospital 

 for criminal insane at Menard. In its typical expression this 

 limestone is thin and moderately thick bedded, the bedding 

 planes being undulating and hummocky in character, with 

 thin shaly partings. In places these shaly partings become 

 thicker and shale beds of as much as five feet or more in 

 thickness are present. The basal portion of the formation, 

 where it is exposed, is seen to be shale, as much as 35 feet 

 of fine, blue clay shale being present in some localities be- 

 tween the top of the Okaw and the typical limestone 

 beds of the formation. The lithologic character of the lime- 

 stone of the formation differentiates the Menard rather sharp- 

 ly from most of the limestone strata of the Okaw. The 

 limestones of the lower formation are commonly more or less 

 crystalline or granular, often crinoidal, sometimes oolitic and 

 usually free from chert. In the Menard the limestones are near- 

 ly always close textured, fine grained rocks, and not infrequent- 

 ly carry a small amount of chert ; they are brittle and often 

 exhibit a conchoidal fracture. Because of the difference in 

 texture the weathered surfaces of the ^Menard are commonly 

 smooth, those of the Okaw usually being more uneven. 

 The color of freshly broken surfaces of the Menard is us- 

 ually a bluish gray while that of the Okaw limestone 

 is commonly lighter, some beds being nearly white. Locally 

 there are more crystaline strata in the ]Menard which closely 

 resemble certain of the Okaw beds, but such strata are 

 always of limited thickness and usually occur in the higher 

 portion of the formation. The thickness of the formation is 

 about 80 feet. It is well exposed in the ^Mississippi River 

 blufiPs from Chester to Rockwood, and the valley of Mary's 

 River, at its mouth, is excavated entirely through this forma- 

 tion into the higher beds of the Okaw. 



8. Palestine Formation. 



General characters. The formation succeding the Me- 

 nard is arenaceous throughout in most sections, consisting 

 in part of heavy beds of sandstone suitable for building pur- 

 poses, and in part of thinly bedded, ripple marked sandstones 

 or arenaceous shales. Locally, however, more argillaceous 

 shales are well developed in the formation. The formation 

 is present in the higher portion of the bluffs at Chester, and 

 has been quarried at several points for building stone. The 

 buildings of the penitentiary at Menard are constructed of 



