744 INDEX TO THE STEATIGKAEHY OF NOKTH AMERICA. 



represents the Oak Grove sand member and the upper fossiliferous marl horizon is the ShoaJ 

 River marl member. Lithologically this member consists of interbedded greenish sands, clays, 

 and marls. The color is "usually greenish, oxidizing yellow, occasionally whitish or purplish, and 

 the material varies in texture from fine clay to sand." This member is about 50 feet thick. 



In Georgia, according to Veatch, ' ' the area over which the Chattahoochee formation appears 

 as a surface formation is small, being confined mainly to small exposures along streams and in 

 lime sinks. 



"Erosion unconformities with the underl3dng Vicksburg have been noted in the vicinity of 

 Bainbridge by Pumpelly and Vaughan. The Alum Bluff foimation everywhere overlies the 

 Chattahoochee and there appears to have been a continuous deposition from the beginning of 

 the Chattahoochee to the end of the Alum Bluff. 



"The formation is, on the whole, calcareous and the rock varies from a compact pure 

 crystalline limestone to earthy and argillaceous limestone and calcareous sand and sandstone. 

 The strata are in places phosphatic, and the limestone at the base has been extensively silicified 

 and contains a rich coral fauna. 



"The formation has a total thickness of about 100 feet in the gorge northwest of Facevdle, 

 Decatur County. No very reliable data are at hand for estimating the thickness east and 

 northeast of this point, where it is under cover of later formations. The thickness is not great, 

 however, and is not hkely to be in excess of 250 feet. 



" The Alum Bluff formation underlies a very large area in Georgia, extending northeastward, 

 from Decatur County to Savannah River in Screven and Effingham counties, the eastern limit, 

 of outcrop being 50 or 60 miles from the coast. This formation overlies the Chattahoochee 

 formation conformably. The line of division between the two is, in places, indefinite, as there 

 is no apparent abrupt hthologic or faunal change from one to the other. In the Savannah and 

 Altamaha river exposures small unconformities between the Alum Bluff and the Miocene were 

 observed. This formation presents several different lithologic phases — sand and sandy laminated 

 clay, fuller's earth, phosphatic sand, quartzite, silicified clay, and local limestone or calcareous 

 layers and nodules. Greenish or drab argillaceous sand and fine-grained sandy laminated clay 

 form the greatest portion of the strata. The maximum thickness of the Alum Bluff in south- 

 western Georgia is estimated at 150 feet. The natural exposures on Savannah River do not 

 exceed 25 or 30 feet. It is not probable that at any place the thickness exceeds 250 or 300 feet, 

 even where it is under cover of the later formations." 



Sediments of upper Oligocene age extend westward, from western Florida to Mississippi 

 River. The Apalachicola group, or marine upper OUgocene, has been identified by means of 

 fossils in Alabama at Roberts and probably at Wallace. In wells at Mobile fossils character- 

 istic of the Alum Bluff formation were encountered between depths of 1,250 and 1,550 feet; 

 below these is limestone, correlated with the Chattahoochee formation. The marine upper 

 Ohgocene is not known west of Mobile, the sediments becoming estuarine in character as the 

 axis of the Mississippi embayment is approached. The unpublished results of the recent field 

 work of Matson and the parallel paleobotanic studies of Berry have shown that the leaf-bearing 

 clays and sandstones near Chicoria, Wayne County, 5 miles south of Florence, Rankin County, 

 and Raglan, near ilcCallum, Perry County, ]\Iiss., are of upper Oligocene age." The exposure 

 at Raglan (the Hattiesburg clays of L. C. Johnson) appears to represent the top of the Alum 

 Bluff of Florida, while the one near Florence is stratigraphically lower and perhaps belongs in 

 the upper part of the Vicksburg group. The exposure of interbedded sandstone, semiquartz- 

 itic sandstone, and clay at Grand Gulf, Miss., is, according to the available evidence, -to be 

 referred to the upper part of the lower Oligocene and is the Mississippi representative of the 

 Catahoula formation of Louisiana at the type locality. These estuarine or fresh-water deposits 

 of clay and sandstone represent the basal portion of the Grand Gulf group of Hilgard,^^*^ which 

 according to his definition included the sandstone and clays lying between the Vicksburg below 

 and the Lafayette above, but which is now known to be a series of geologic formations, including 



= Paleontologic determinations furnished by E. W. Berry. 



