UPPER CRETACEOUS. 657 



prevailed in the region of the Chattahoochee Valley in eastern Alabama and western Georgia 

 were, during much of Cretaceous time, markedly different from those existing farther west. 



The lowest Cretaceous division of the Chattahoochee region, the Tuscaloosa group of Lang- 

 don,^^° was supposed by that author to represent the eastward extension of the Tuscaloosa of 

 Smith and Johnson. The physical aspects and relations of these deposits, and the paleontologic 

 evidence furnished by a collection of poorly preserved fossil plants from Old Fort Decatur, on 

 Tallapoosa River, studied by E. W. Berry, have led both Mr. Berry and the writer to doubt the 

 correctness of this correlation. It is believed that they are of Lower Cretaceous age and hence 

 older than the Tuscaloosa formation. 



The Eutaw group of Langdon in the Chattahoochee region is approximately the eastward 

 extension of the Eutaw of Smith and Johnson except that in the opinion of the writer, based on 

 paleontologic evidence, it should have included at least 120 feet of the overlying beds which 

 Langdon referred to the base of his Ripley group. The beds of the division exposed on Chat- 

 tahoochee River are in the main of marine origin, although in places they exhibit in part the 

 characters of shallow near-shore or estuarine deposits. To the northeast in Georgia they pass 

 into shallow-water equivalents and eventually end unconformably against the underlying older 

 terrane (Lower Cretaceous). 



The conditions which favored the deposition of chalkhke rocks in central and western 

 Alabama were entirely lacking in the Chattahoochee region, and we have in place of the chalk 

 a series of marine sands and clays which as a whole are synchronous with the fully developed 

 Selnia chalk of western Alabama. The conditions unfavorable to the deposition of chalk, 

 which at the beginning of Selma time existed only from Macon County eastward, gradually 

 spread westward as Upper Cretaceous time proceeded, ending, so far as the record has been 

 preserved, a short distance east of Tombigbee River in Marengo County, Ala. These nonchalky 

 deposits of the Chattahoochee region were by Langdon referred in their entirety to his Ripley 

 group. Recent studies by the writer in this region have shown that Langdon's Ripley group of 

 the Chattahoochee region is divisible on paleontologic grounds into two parts — a lower part 

 carrying a number of species restricted to the lower one-third or one-haK of the formation, and 

 an upper part carrying a large number of species that do not range lower than the upper one-half 

 or two-thirds of the formation, and characterized especially by the presence of Exogyra costata 

 Say. These two parts, with the exception of the Tombigbee representatives at the base of the 

 lower one, correspond to the two parts recognized paleontologically in the Selma chalk. There 

 are, however, in the Chattahoochee region, a large number of wide-ranging species common to 

 both the lower and upper divisions noted. 



Northeastward from Chattahoochee River in Georgia the equivalents of Langdon's Ripley 

 group pass first into a series of alternating marine and shaUow-water beds, and finally, still far- 

 ther to the northeast, into irregularly bedded sands and clays of shallow-water origin. The 

 latter are overlapped and concealed in central and eastern Georgia by Eocene beds. 



The classification of the Georgia Cretaceous deposits adopted by Otto Veatch ^" in 1909, is 

 essentially the same as that of Langdon, except that Veatch subdivided the Ripley into four 

 parts on the basis of the alternation of beds of marine and shallow-water origin recognizable 

 in a part of the area. These subdivisions are, in ascending order, the Blufftown marl, Cusseta 

 sand, Renfroes marl, and Providence sand. 



The lower part of the "Blufftown marl" is probably synchronous with the Tombigbee sand 

 member. The upper part of the "Blufftown marl," the Cusseta sand, the so-called Renfroes 

 marl, and the Providence sand together are the equivalents of the Selma chalk where that 

 formation is most fully developed in western Alabama and east-central Mississippi. 



As regards the correlation of the Upper Cretaceous deposits of the eastern Gulf region with 

 deposits in regions outside of this area, the following may be said : That part of the series charac- 

 terized by the presence of Exogyra costata Say, which includes the Ripley formation of northern 

 Mississippi; about the upper half of the Selma chalk where most fully developed; and the upper 

 one-haK or two-thirds of the Ripley formation as developed in the Chattahoochee region are 



48011°— 12 42 



