1 22 ILLINOIS ACADEMY OF SCIENCE 



most localities is a darker brown than that of freshly broken 

 surfaces. W'herever the streams have eroded their chan- 

 nels in this massive sandstone formation, picturesque gorges 

 have been formed with nearly vertical walls. The maximum 

 thickness of the formation probably does not exceed 70 or 

 80 feet, and practically the entire maximum thickness is ex- 

 posed in the Mississippi river bluffs, two miles above Modoc, 

 and in the bluffs of Rock House Creek, three and one-half 

 miles southeast of Waterloo. To the eastward, north of 

 Prairie du Rocher, the formation thins out entirely, the super- 

 jacent Renault formation resting, by overlap, directly upon 

 the St. Louis limestone. 



Sub-Brewerville unconformity. Throughout the 

 Randolph-Monroe county area, the Brewerville rests uncon- 

 formably upon the underlying formations. The subjacent 

 limestone floor in most of the area is the St. Louis limestone, 

 but in the Mississippi River bluffs below Prairie du Rocher, 

 and in some of the ravines southeast of the same town, a 

 remnant of the Ste. Genevieve limestone is still present. The 

 demonstration of the unconformity, however, is not alone 

 dependent upon the absence of the Ste. Genevieve in part of 

 the area. At the close of the erosion period during which the 

 Ste. Genevieve limestone was wholly removed over a large 

 area, the resultant surface was uneven, although with no ab- 

 rupt relief features. Where the actual contact of the Brewer- 

 ville upon the underlying strata can be seen, as in the bluffs 

 between Prairie du Rocher and Modoc- the difference in 

 elevation of the contact, as well as the different strata at the 

 summit of the subjacent formation, are clearly discernable. 

 A difference in elevation of the contact line, of at least 100 

 feet in a distance of one mile or less, is observable west of 

 Red Bud. Furthermore, at many localities on the north fork 

 of Horse Creek and its tributaries, and in Rock House Creek, 

 the basal bed of the Cypress is a conspicuous layer of brec- 

 cia, one foot or more in thickness, made up of angular masses 

 of the very characteristic chert from the upper St. Louis 

 limestone beds, these masses ranging in size from a fraction 

 of an inch to a foot or more in the their maximum dimension. 



2. Renault Formation. 



General characters. The Renault formation consists of 

 an exceedingly complex series of strata whose lithologic 

 characters change rapidly both in horizontal extent and 

 vertically. The formation includes sandstones, arenaceous 

 shales, variegated green, blue and purple shales, calcareous 

 shales, thin platy layers of limestone in some of the calcare- 

 ous shales, dense arenaceous limestones, nearly pure crystal- 

 line limestones, oolitic limestones, and in the northern part of 



