4 GEOLOGY OF ILLINOIS. 



the river bank. A section of this bluff, made nearly midway between its north- 

 ern and southern extremities, shows the following order : 



FEET 



Loess capping the bluff <*** 



Light-gray Trenton limestone 8 



Brown and buff magnesian limestone TO 



St. Peters Sandstone 13 



The sandstone is irregularly stratified, and often concretionary, showing no 

 well defined lines of bedding. It may be regarded as the equivalent, in part, 

 of the Calciferous group of New York, and corresponds to the Saccharoidal 

 sandstone of Missouri. No organic remains have yet been found in it, either 

 in this State or elsewhere. 



Trenton Group. This group, as it is developed in this county, consists of 

 brown and buff magnesian limestones at the base of the series, which attain a 

 thickness of about seventy feet. These are succeeded by fine grained, compactj 

 gray and chocolate brown limestones forming the middle divisions of the series 

 and these are overlaid by a rather soft, coarse grained, yellowish gray limestone, 

 forming the upper division of the group. Its aggregate thickness may be estima- 

 ted at three hundred and fifty to four hundred feet. Its most northerly outcrop 

 in this county, is at the foot of the Mississippi bluffs, about three miles below 

 Gilead P. 0., on the northeast quarter of section 31, town 11, range 2 west, 

 though it was found only about three feet below the surface in digging a well 

 on the southwest quarter of section 29, in the same township. The rock where 

 it first appears in this vicinity, is a light yellowish gray, coarse grained lime- 

 stone, rather soft and very uneven in texture, and weathers on exposure with 

 an uneven and ragged surface. It is rather thin bedded at the top, but be- 

 comes more massive below, and the strata rise in a southerly direction so rapidly 

 that about two miles below the point where the rock first appears, it forms a 

 perpendicular cliff from eighty to a hundred feet in hight. A few fossils were 

 obtained from these coarse grained limestones, among which were Strophomena 

 alternata, Ortliis lynx, and a ramose form of Chsetetes. Below this coarse 

 grained limestone, we find about fifteen or twenty feet in thickness of fine 

 grained, chocolate colored, thin bedded limestone. It weathers to an ash gray 

 color, and the strata are generally from two to four inches in thickness. 



Descending along the river bluffs below the outcrop of these limestones, we 

 find them underlaid by a series of light gray, compact, fine grained limestones, 

 partly thin bedded, but affording some massive strata in the lower part of the 

 series. These limestones continue to form the main portion of the river bluff 

 down to the small creek which intersects the bluffs just above the Cap au 



Ores ferry landing. At this point the upper layers of the brown and buff 

 limestones, which form the lower division of the group, are seen just above the 



creek level. 



