EARLIEK TERTIAEY (EOCENE AND OLIGOCENE). 739 



The formation has a low southward dip and the width of the outcrop is only a few miles, so 

 that the total thickness probably does not exceed 150 or 200 feet." 



On the South Carolina side of Savannah River, at Johnson's Landing, are exposures of 

 silicified limestone which is replete with Bryozoa and contains a few specimens of Peden per- 

 planus. This rock may belong to the Jackson, but in this region the exposures are poor and 

 fossils are rare and not well preserved, rendering it difficult to discriminate between Jackson, 

 Vicksburg, and Chattahoochee. 



The thick beds of soft limestone exposed along Ashley and Cooper rivers in the vicinity 

 of Charleston, called by Sloan '^*® the Ashley-Cooper marl, are referable to the Jackson group. 

 The marl is green-drab or gray-green and slightly plastic when wet, but is lighter in color and 

 more pulverulent when dry. The marl along Cooper River usually contains less than 2 per 

 cent of calcium phosphate; that along Ashley River may contain as high as 15 per cent. The 

 formation exceeds 100 feet in thickness. 



OLIGOCENE. 



Vicksburg group. — This group takes its name from the city of Vicksburg, Miss., in the 

 vicinity of which are excellent exposures, especially along the bluffs of Mississippi River. The 

 outcrops of the group extend from Louisiana to Savannah River, except where they are inter- 

 sected by stream valleys. In Mississippi it consists of a semicrystalline limestone in beds 

 varying from 1 foot to 3 feet in thickness, alternating with beds of sandy fossiliferous marl 

 of about the same thickness. Usually the beds of limestone near the surface are more or less 

 affected by weathering and have therefore become soft and locally yellow, but below the zone 

 of weathering there is in many places hard blue rock. The beautiful, well-preserved fossils 

 of the Vicksburg are found mostly in the beds of marl. The thickness of the formation is between 

 65 and 75 feet in Mississippi. This formation marks the end of the marine conditions of the 

 Tertiary in western Mississippi. Its outcrop occupies a narrow band just south of the Jackson 

 area and extends across the State from the type outcrop at Vicksburg through Warren, Hinds, 

 Rankin, Smitl;i, Jasper, and Wayne counties into Alabama. 



In eastern Mississippi near Hiwannee, on Chickasawhay River, the base of the Vicksburg, 

 immediately overlying the Jackson, bears fossils of an older facies than those at Vicksburg and 

 constitutes the Red Bluff group of Hilgard, which is composed of calcareous clays and some 

 limestone beds. 



The Vicksburg continues across southern Alabama, where it has not been differentiated 

 from the Jackson (uppermost Eocene), the two being included in the St. Stephens limestone. 

 It seems, however, that by more detailed studies the Vicksburg and Jackson can be discriminated. 

 The Vicksburg part of the St. Stephens limestone consists of a limestone of a considerable degree 

 of purity, in which the nummuUtoid fossil Orbitoides is persistently present. The rock is; 

 largely a foraminiferal limestone. Other fossils are present, but this one is characteristic. In 

 some places this limestone is hard and capable of pohsh, but usually it is soft and easily cut 

 with a saw. Its thickness is between 200 and 300 feet. It outcrops across Clarke, Choctaw 

 and Washington counties and extends eastward into Georgia and southward and southeastward 

 into Florida. 



In Georgia the Vicksburg, according to Veatch, "consists mainly of white, massive-bedded 

 limestone interbedded with sand and clay. The limestones are extensively silicified and in 

 many places do not appear at all at the 'surface but are concealed by red coarse-grained 

 ' argillaceous sand and are represented by residual flint. The limestone is generally soft but in 

 a tew places is exceptionally hard and crystalline and very fossiliferous, Orbitoides, Bryozoa 

 pectens, and their fragments forming nearly the whole of the rock. 



"The thickness of the Vicksburg in the western part of the State, to judge from well records 

 is hardly more than 300 feet, and there is no evidence that it is likely to exceed this at the east. 



" The Vicksburg formation in Georgia underlies a large area lying mainly west of Flint River. 

 East of Oconee River the formation is usually concealed by later formations and in Screven 

 and Burke counties only small areas are known." 



