92 CEMENT MATERIALS AND INDUSTRY. [bull. 243; 
The eastern limit of the limestone outcrop on the south side of White 
River is in the NW. i sec. 5, T. 14 N., R.. 8 W., just above Renters 
Bluff. Opposite the bluff the limestone horizon is concealed by chert 
debris. Upstream from the outcrop in sec. 5 the hills became steeper, 
and are so close to the river that from Renters Bluff to the mouth of 
Sylamore Creek they form a river bluff, which is broken b} r numerous 
small creeks and ravines and by two short strips of alluvium — Jones 
Bottom, in R. 9 W., and Round Bottom, in R. 10 W. This bluff is 
not so high nor so prominent as Renters Bluff, but it consists of the 
same rocks — Izard limestone at the base, overlain by Rolk Bayou 
limestone, which is capped with chert. 
STRATIGRAPHIC POSITION OP POLK BAYOU AND ST. CLAIR LIMESTONES. 
: 
Stratigraphic position. — The St. Clair limestone and Rolk Bayou fo 
mations, considered together, form one of the thickest and most impo 
tant beds of limestone in the State. They are underlain by the blue 
Izard limestone and overlain by the Sylamore sandstone (Devonian) or] 
the Chattanooga shale, one or both of which are generally present, 
often in an inconspicuous bed only a few inches in thickness. In the 
absence of both the Sylamore sandstone and the Chattanooga shale the; 
St. Clair-Rolk Bayou limestones are overlain by the St. Joe limestone, 
which forms the base of the Boone formation. 
THICKNESS OF THE ST. CLAIR-POLK BAYOU LIMESTONES. 
The maximum thickness, which is 155 feet or more, is at Penters 
Bluff, on White River. The limestone thins out gradually toward the 
east, west, and north; on Polk Bayou it is probably not more than 
100 feet thick, while on Dota Creek, still farther east near the Paleo- 
zoic border, it does not occur at all. Above the mouth of Hidden 
Creek, on White river, it is 50 feet thick; but a few miles farther up, 
the river, below the mouth of Twin Creek, there is only a trace of it. 
On the south side of White River, on Little Rocky Ba} 7 ou, its thick- 
ness is from 25 to 40 feet; on South Sylamore it is from 25 to 50 feet, 
and at St. Joe it is from 20 to 30 feet. 
DESCRIPTION. 
In general both the Polk Bayou and the St. Clair limestones are highly 
crystalline, being composed of small crystals of nearly uniform size. 
They are tenacious, easily cut, break with difficulty, and have 1 
slightly conchoidal fracture. In weathering, the crystals are sepa- 
rated, resembling coarse sand. 
These formations commonly outcrop in heavy layers from 2 to 4 
feet or more in thickness; but in some places the rock is massive, the j 
entire exposure being in one solid bed. 
