CALHOUN COUNTY, 5 



On Cave Spring Branch, a small creek which intersects the bluffs about a 

 mile and a half above the ferry landing, the upper portion of the Trenton lime- 

 stone is well exposed, forming the bed and bluffs of the creek for a mile or more 

 from its mouth. The thin bedded, chocolate colored limestone, is also well exposed 

 on this creek, and is here quite arenaceous and passes into a fine grained calca- 

 reous sandstone. Some of the light gray compact limestones below this choco- 

 late colored bed, are filled with marine plants, or fucoides, which are well ex- 

 posed on the weathered surfaces of the rock. Trilobites are not uncommon in 

 these limestones, and fragments of Asaphus megistos, Ceraurus pleurexanthemus, 

 and lllenus ovatus were obtained here. They are associated with two or three 

 species of Orthocera, and the common Brachiopoda of this horizon. 



The lowest division of this group consists of evenly bedded, buff or brown 

 dolomitic limestones, which attain a thickness of about seventy feet, and are 

 seen overlying the St. Peter's sandstone at the upper end of the Cap au 

 Ores bluff, the only point where they are found well exposed. The beds vary 

 in thickness from four inches to two feet or more, and the dip is so strong to 

 the northeastward, that this division of the group only outcrops over a very 

 limited area, in the immediate vicinity of the river bluffs. Indeed, the whole 

 of this group, nearly or quite four hundred feet in thickness, covers in its out- 

 crop in this county, an area of only about three or four square miles. 



Cincinnati Group. The Trenton limestone is immediately overlaid in this 

 county, by blue and green, partly indurated clays, which attain an aggregate 

 thickness of about a hundred feet, and although they have afforded no fossils, 

 their stratigraphical position, and lithological characters, are sufficient to deter- 

 mine their position in the geological series, as the equivalents of the Cincinnati 

 group of our general section of the Illinois strata. These clays are seldom 

 found well exposed, but partial outcrops are occasionally seen on the slopes of 

 the hills, either on the small streams or gulches, which intersect the river bluffs 

 along their line of outcrop. They are often met with in digging wells in the 

 region which they underlie, and where the Upper Silurian limestone is want- 

 ing, this group forms low rounded hills, or gentle slopes, that seldom afford any 

 good exposure of the underlying strata, although they may be but a few feet be- 

 neath the surface. Their first appearance on the eastern side of the county 

 on descending the river bluffs, is between Hamburg and Gilead, where they are 

 occasionally seen cropping out beneath the Niagara limestone, which here forms 

 the upper part of the bluff. When exposed at the surface, they form a tough, 

 blue plastic clay, very much like the potter's clays of the coal formation. On 

 the northwest quarter of section 19, township 10, range 2, west, they outcrop 

 beneath the Niagara limestone, and extend down to the river level and below, 

 showing a thickness above the river, of about forty or fifty feet. Gradually 

 rising in a southerly direction, they are found in the vicinity of Gilead about 



