COOKE : JACKSON AND VICKSBURG DEPOSITS 195 



Clypeaster rogersi. From a thickness of 74 feet at St. Stephens 

 Bluff, Alabama, the "chimney rock" thins to about 45 or 50 

 feet on Chickasawhay River, Mississippi, and to about 20 feet 

 in the neighborhood of Brandon, Mississippi. 



Glendon limestorie memher. Overlying the ''chimney rock" 

 and conformable with it is a series of ledges of hard, partly 

 crystalhne, yellowish or pinkish limestone interbedded with 

 softer strata of impure limestone composed largely of Bryozoa, 

 Foraminifera, and shells of Ostrea vicksburgensis and Pecten 

 poulsoni. This rock is distinguished from the other parts of the 

 Marianna limestone mainly by its lithology, but a few species of 

 organisms are restricted to it. At Glendon, Alabama, it is 18 

 or 20 feet thick and overlies 20 feet of "chimney rock." 



The Glendon limestone extends from McGowans Bridge, 

 Conecuh River, to Mississippi River at Vicksburg. It forms the 

 hard ledges at the top of St. Stephens Bluff and the cap rock 

 of several picturesque waterfalls near Vicksburg. Although it is 

 in few places thicker than 20 feet, the Glendon limestone, because 

 of its hardness, is the most conspicuous part of the Vicksburg 

 group in Mississippi, to which it has given the undeserved 

 reputation of being composed chiefly of limestone. 



Mint Spring calcareous marl member. The "chimney rock" 

 facies of the Marianna limestone is replaced in western Alis- 

 sissippi by sands and shell marls for which the name Mint Spring 

 calcareous marl is here proposed. The name is derived from 

 Mint Spring Bayou, a small stream entering Centennial Lake 

 just south of the National Cemetery at Vicksburg. The strata 

 to which the name is applied are exposed beneath a waterfall 

 in the lower course of the stream. 



Between Vicksburg and Pearl River the Mint Spring marl 

 occupies the entire interval between the Forest Hill sand and 

 the Glendon limestone, but east of Pearl River, it is overlain by 

 a thickening wedge of the Marianna "chimney rock." It has 

 not been recognized east of Chickasawhay River, on which 

 it is exposed Ij miles northwest of the mouth of Limestone 

 Creek. Other important exposures are along Glass Bayou at 

 Vicksburg, and at Haynes Bluff, 14 miles north of Vicksburg, 

 where it is 25 feet thick. 



