16 CRETACEOUS DEPOSITS OF THE EASTERN GULF REGION. 



Lithologic subdivisions. — In 1903 Smith 1 divided the SeJma chalk in Alabama into three 

 parts on the basis of the relative content of lime and clay. The lower portion, which he esti- 

 mates to include approximately one-third of the total thickness of the formation, contains 25 

 per cent or more of clayey impurities ; this he called the Selma division. (See PL V, B.) The 

 middle portion, estimated to embrace one-third of the total thickness, contains less than 25 

 per cent of clayey impurities; this he called the DemopoJis division. (See PL V, A.) The 

 upper portion, embracing the remahider of the formation, contains 25 per cent or more of 

 clayey impurities; this he called the Portland division. 



The purer phase of the chalk, Smith's Demopolis division, is traceable from western Ala- 

 bama eastward in Alabama and northwestward and northward in Mississippi, but in each 

 direction it becomes gradually thinner and less pure and eventually grades into impure sandy 

 and argillaceous phases of the chalk rock. The impurities mentioned by Smith as distinguishing 

 the lower and upper divisions of the chalk are not all of a clayey character, for important per- 

 centages of sandy impurities are known to be present in considerable thicknesses of the strata, 

 especially in the upper division in western Alabama and east-central Mississippi. 



Eastward in Alabama and northward in Mississippi the Selma chalk merges along the 

 strike of the beds into nonchalky equivalents. This relation, as regards eastern Alabama, 

 was formerly recognized as probable by the Alabama geologists (as shown by the quotations 

 given below), but the vertical ranges of the fossils were not at the time sufficiently well known 

 to permit a positive statement. 



It will be seen that the main variation from the western Alabama type consists in thev ery great increase in the 

 area occupied by the strata of the Ripley type in the eastern part of the State. Whether this results in part from an 

 increase in the thickness of the strata themselves or from undulations in them may not perhaps be definitely asserted, 

 but, taken in connection with the apparent absence of all the strata that can be referred to the Rotten limestone, it 

 seems most probable that there is actually much greater thickness of the rocks of the Ripley type along the Chatta- 

 hoochee and its vicinity than farther west, and that the Rotten limestone is replaced or represented'by strata of the 

 physical aspect of the Ripley. The paleontology of these two divisions of the Cretaceous has not been very well 

 worked out, so that the shells give us comparatively little help in the matter, especially when we consider the fact 

 that in the Rotten limestone, although it has a very large number of shells in its strata, these shells are of very few 

 kinds and mostly of those kinds that are common in the Ripley strata also. 2 



East of the drainage of the Alabama River the Rotten limestone, such as occurs in Marengo, Perry, Dallas, Lowndes, 

 and Montgomery counties, is not represented. The exact eastern limit of this group has not as yet been determined, 

 but evidences of its decreasing thickness are seen in the narrow outcrop in the neighborhood of Pike road, Mont- 

 gomery County, where its north and south extent is only 5 miles as contrasted with 30 miles in Dallas County. Further 

 than this decrease in thickness our present information does not warrant us in saying anything. As has been stated 

 before, no rocks bearing any lithologic resemblance to the Rotten limestone have been seen on the Chattahoochee 

 River, whether or not this group is represented by strata of different composition from the typical aluminous limestone 

 we are not in position to say, since no critical examination of the fossils of the several divisions of the Cretaceous 

 has yet been undertaken. It Is much to be regretted that the divisions have been of necessity made on such arbitrary 

 grounds as mere lithologic differences, since marked variations can be noted in almost any stratum of any of the 

 groups, and experience in both the Tertiary and Cretaceous of Alabama has proved the risk of creating groups on 

 any but combined physical and faunal differences. 3 



Deposition. — When the formation of the Sehna chalk began, the conditions favorable to 

 this sort of deposition existed throughout an offshore area extending from western Russell 

 County, Ala., to Itawamba County, Miss., a distance of approximately 300 miles. As Upper 

 Cretaceous time progressed the area in which chalk was being laid down underwent certain 

 marked expansions and contractions due to fluctuating conditions of sedimentation. In 

 Mississippi conditions unfavorable to the deposition of chalk at first spread gradually south- 

 westward from Itawamba County for 15 or 20 miles, producing the tongue of Tombigbee sand 

 which extends from the north to the south through Lee County. (See PL IX, in pocket.) 

 This was followed by a rapid northward extension of conditions favorable to the deposition of 

 impure chalk, thus producing a long tongue of Selma chalk reaching as far as Hardin County, 

 Tenn. Then succeeded a more gradual spreading of unfavorable conditions southward through 



1 Smith, E. A., The Portland cement materials of central and southern Alabama: S. Doc. Xo. 19, 5Sth Cong., 1st sess., 1903, pp. 12-23, map. 

 ' Smith. E. A ., Langdon, D. W., jr., and Johnson, L. C, On the geology of the Coastal Plain of Alabama, Geol. Survey Alabama, 1894, pp. 

 275, 276. 



» Idem, pp. 430, 431. 



