PIKE COUNTY. 29 



the county as on the other. From this point northward, these shales appear in 

 occasional outcrops along the bluffs, to the vicinity of Bedford, where they dip 

 below the surface and are seen no more. As this group does not form the bed 

 rock over any considerable surface area in this county, it fails to impart any of 

 the peculiar topographical features to the surface here, which usually character- 

 ize it where it is well developed, with no overlying limestone to modify its in- 

 fluence on the topography of the country. Then, it almost invariably forms a 

 broken and hilly region, so marked in its character that the extent of its out- 

 crop may be very accurately defined, from the peculiar topographical features of 

 the surface alone. But almost everywhere in this county, where the group is 

 exposed, the Burlington limestone overlies it, and therefore determines the 

 topographical features of the region also underlaid by the shales and gritstones 

 of this group. 



Burlington Limestone. This limestone forms the bed rock over fully one-half 

 of the entire surface of the uplands in this county, and its outcrop, in a gene- 

 ral way, may be thus described : Commencing on the western side of the north 

 line of the county, it forms a belt, from five to ten miles in width, the 

 western border of which is defined by the river bluffs, and extending thence to 

 the southern line of the county, forming the bed rock over all that part of the 

 county lying south of Pittsfield, and from that point northward to Griggsville 

 Landing, and south to the Calhoun county line, underlying all the highlands in 

 that portion of the county south of Pittsfield, except a very limited area in 

 the vicinity of Pleasant Hill, where the Niagara limestone forms the surface 

 rock. Its thickness ranges from one hundred and fifty to two hundred feet, but 

 usually not more than fifty or one hundred feet can be seen at a single outcrop. 

 The best exposures of this rock are to be seen in the bluffs of the Illinois and 

 the Mississippi, and on some of the principal creeks in the western and south- 

 ern portions of the county. The rock is a rather coarse grained, gray lime- 

 stone, with intercalations of buff or brown layers, and is largely composed of 

 the fossilized remains of the Crinoidea and Mollusca, that swarmed in countless 

 myriads in the old carboniferous ocean, during the formation of this limestone. 

 It is the Crinoidal and Encrinital limestone of some of the old observers, and 

 it was so designated in consequence of its being almost entirely composed, at 

 some localities, of the remains of these radiated forms of animal life. Indeed, 

 the main portion of the rock consists of the calcareous plates and joints of 

 crinoids, with barely enough mineral matter to cement the organic remains to- 

 gether. 



In the Mississippi bluff, near the north line of the county, there is from 

 forty to fifty feet of the lower portion of this limestone exposed, forming the 

 upper escarpment of the bluff. These lower beds consist of alternations of 

 gray and brown limestone, usually in regular and tolerably thick beds, and con- 



