80 



SHORTER CONTRIBUTIOXS TO GENERAL GEOLOGY, 1!)21. 



and corals shows that thi> marl at Byraiii is of 

 the same ago as the upper shell bed at Vicks- 

 burij, and this correlation is entirely corro- 

 orated hy the I'vidence of the Bryozoa and the 

 Foraminift'ra. 



The Byrani marl lies conformably upon the 

 Glendon limestone member of the Marianna 

 limestone. The relations of the Byram marl 

 to the overlying Catahoula are conjectural. 

 At Vicksburg the transition from one formation 

 to the other is so gradual that deposition ap- 

 pears to have been continuous from the Vicks- 

 burg into the Catahoula, but at Waynesboro 

 th(! change in lithology is so abrupt as to sug- 

 gest the probability of an interruption in 

 deposition. However, no uneciuivocal evi- 



At Vicksburg, where the entire thickniess of 

 the Byram marl is exposed, the formation is 

 only 42J feet thick. On Chickasawhay River, 

 in eastern Mississippi, incomplete exposures 

 indicate a thickness of at least 70 feet. Because 

 of the softness of the Byi-am marl, exposures 

 are few and usually incomplete, and in many 

 places the whole formation is covered. The 

 outcrop of the Byram marl extends entirely 

 across the State of Mississippi. Exposures arc 

 known on Mississippi, Pearl, Leaf, and Chicka- 

 sawhay rivers and at a few intermediate 

 places, and strata of the same age have been 

 found at several places in Alabama. 



No marked physiographic features are duo 

 to the Byram marl. The soft, easily eroded 



However, no unecpiivoca 



Correlation of thr Jnckson iniil Virlshiirij lUposits in Mississippi and Alahninn 



dence of unconformity between the two for- 

 mations has been observed. 



Although the Byram marl, bcung the forma- 

 tion from which Com'ad obtained most of his 

 fossils from \'icksburg and upon which he 

 based his di>scription of the Vicksburg group, 

 is the type formation of the group; it is not 

 nearly so conspicuous as the Marianna lime- 

 stone, which underlies it, and this has given 

 rise to the erroneous impression that the Vicks- 

 burg group consists chiefly of limestone. The 

 formation, as the name indicates, consists 

 largely of nuirl, but it contains also thin beds 

 of impure limestone, clay, and sand. The 

 marl is generally sandy and contains con- 

 siderable glauconite. On natural outcrops it 

 is of a rusty yellowish color, but where less 

 weathered it is gray or green. 



strata usually form gt-ntle slopes between the 



harder Cdendon limestone member of the 



Marianna limestone, below, and the Catahoula 



sandstone, above. At some places the Byram 



marl has been eroded back from the edge of the 



Glendon limestone, leaving the hard ledges of 



the Glendon projecting as a broad platform 



or terrace 



LOCAL DETAILS. 



VICINITY OF VICKSBURG. 



Exposiu'es of the Byram marl occur at 

 several places near Vicksburg. The best and 

 most complete is on the Park Road leading 

 southward up the hill from Waltersville past 

 the north side of the National Centctery. 

 Bed 1 of the following section, which shows 

 the entire thickness of the Byram marl as well 

 as parts of the adjacent formations, was found 



