86 



THE ST. STEPHENS LIMESTONE. 



General Description. The St. Stephens or White limestone 

 formation of the Alabama Tertiary, which includes the upper- 

 most of the Eocene strata, is in general equivalent to the Vicks- 

 btirg limestone of the Mississippi geologists. 



In Alabama it exhibits three rather well-defined phases, 

 which in descending order are ( I ) the Upper or Salt Motmtain 

 division, observed at one locality only in Clarke county; (2) 

 the Middle or St. Stephens division, and (3) the .Lower or 

 Jackson division. Of these it is only the middle division with 

 which we are here concerned, since the first is, so far as known, 

 restricted to one locality, and the third is seldom exposed along 

 Alabama rivers and railroads. 



The following section of the St. Stephens Bluff, Tombigbee 

 River, (Plate VI), will give an idea of the strata of this di- 

 vision : 



Section of St. Stephens Bluff. 



FEET. 



1. Red residual clay 1 to 5 



2. Highly fossiliferous limestone holding mainly oysters, 



and full of holes, due to unequal weathering 10 to 12 



3. Orbitoidal limestone (chimney rock), a soft, nearly uni- 



form porous limestone, making smooth perpendicular 

 face of the bluff except where bands of harder lime- 

 stone of very nearly similar composition alternate with 

 the softer rock. Both varieties hold great numbers of 

 the circular shells of Orbitoides mantelli. These harder 

 ledges are nearly pure carbonate of lime, take a good 

 polish, and are often burned for lime 60 



4. Immediately below 3, for 5 or 6 feet, the strata were not 



visible, being hidden by the rock falling from above, 

 but the space seems to be occupied by a bluish clay. 

 Then follows a soft rock somewhat of same consistency 

 as No. 3 above, but containing a good deal of green 

 sand. The fossils are mostly oysters and P/m//oxf<mm 

 dumosa. This bed is in places rather indurated super- 

 ficially, and forms projecting ledges 10 to 15 



