188 COOKE : JACKSON AND VICKSBURG DEPOSITS 



called the Yazoo clay member and the Moodys calcareous marl 

 member of the Jackson formation, of which the Yazoo clay is 

 the upper and the Moodys marl the lower member. Although 

 the typical exposures of the Yazoo clay are in the bluff of Yazoo 

 River at Yazoo City, Mississippi, both of these members crop 

 out in the vicinity of Jackson, Mississippi, where their relative 

 stratigraphic position is evident. The Moodys calcareous marl 

 member is named from Moodys Branch, a small tributary of 

 Pearl River within the city limits of Jackson. 



The Jackson formation in Mississippi is composed chiefly of 

 more or less calcareous clay and less prominent sand and marl 

 beds. At Jackson and Garland's Creek, the Moodys marl 

 member contains at the base a bed of shells inclosed in quartz 

 sand and glauconite and merges below into lignitic clay and 

 sand supposed to be of upper Claiborne age (Yegua formation) . 

 Toward the top, the Moodys member is less sandy and much 

 more calcareous and contains thin beds of indurated marl or 

 impure limestone. Although these ledges of marlstone are dis- 

 continuous, the zone in which they are found extends from 

 Yazoo River to western Alabama, where it has been called the 

 " Zenglodon bed."^ The Yazoo clay member consists almost 

 entirely of calcareous, very plastic clay of various colors, but in 

 most places blue or green when wet but gray when dry. 



The thickness of the Jackson formation varies considerably 

 from place to place. In general, the Moodys marl thins from 

 east to west and the Yazoo clay thickens rapidly in the same 

 direction. In central and western Mississippi, its thickness 

 does not much exceed 35 or 40 feet, but farther east, owing to 

 the interpolation of beds of sand and clay, it is materially greater. 

 In Alabama, a few miles east of the Mississippi state line, the 

 equivalents of the Moodys marl are more than 90 feet thick. 

 The Yazoo clay member is thickest in the extreme western part 

 of Mississippi, where well borings south of Vicksburg indicate a 

 thickness of nearly 600 feet, and thins rapidly toward the east. 



5 ScHUCHERT, Charles, U. S. Nat. Mus. Proc. 23: 329. 1900; Cooke, C. 

 W., U. S. Geol. Survey Prof. Paper 95: 116. 1915. 



