6 GEOLOGY OF ILLINOIS 



one hundred feet or more in thickness, but seldom well exposed. From this 

 point, their outcrop trends southeastwardly across the county, following the 

 direction of the Cap au Ores fault, and appearing in the Illinois river bluffs, 

 on the eastern side of the county, in the vicinity of Monterey, where they form 

 the base of the bluff, and are overlaid by the Niagara and Hamilton limestones. 

 From this point northward, they are occasionally seen at the base of the bluffs 

 for two or three miles, when, with a gentle northeasterly inclination, they pass 

 below the level of the Illinois bottoms, and are seen no more. No calcareous 

 or arenaceous strata, were found associated with this group in this county, and 

 it appears to be composed entirely of fine argillaceous sediments. 



Niagara Limestone. This is one of the most important formations in the 

 county, and is well exposed at many points in the river bluffs, on both the east 

 and west sides of the county. On the west, it appears at the base of the bluff 

 near the north line of the county, forming a low bench of light gray limestone, 

 from fifteen to twenty feet above the river level, and thence extends down 

 nearly to the mouth of Bay creek, where, by an indulation of the dip, it sinks 

 below the level of the river, and does not appear again above Hamburg. At 

 that point it again rises above the river level, and at the mouth of the small 

 creek, which enters the river on the lower side of the village, there is an out- 

 crop of the upper part of this formation, about twenty feet in thickness, over 

 which the creek forms a cascade just above its mouth. The rock is here a 

 compact bluish gray limestone in regular beds, from six inches to a foot in 

 thickness. It has a moderate dip to the northward, and a half mile below this 

 point, where another creek enters the river, a measured section showed about 

 forty-five feet of this limestone above the river level. At all the localities 

 where this formation was seen, from the north line of the county to a point 

 some two miles below Hamburg, the rock is of a bluish gray color, and usually 

 even bedded, but south of this, it changes to a light brown or buff color, and 

 presents the characters of a true dolomite. In the vicinity of Hamburg it is 

 immediately overlaid by a brownish gray, arenaceous, Devonian limestone, and 

 this is succeeded by the limestones and shales of the Kinderhook group. The 

 following section will show the relations of these different formations as they 

 appear in the vicinity of Hamburg, including all, to the highest point of the 

 bluff: 



FEET. 



Loess forming the summit of the bluff 60 



Burlington limestone 40 



Shaley ash colored limestones (Kinderhook) 40 



Greenish sandy and argillaceous shales (Kinderhook) 60 to 70 



Slaty oolitic limestones " 10 to 15 



Fine grained light blue limestone " 4 to 20 



Green shale 1 to 3 



Hamilton limestone (Devonian) 4 to 8 



Niagara limestone (Up. Silurian) 40 to 50 



