810 INDEX TO THE STKATIGRAPHY OF NOETH AMERICA. 



pebbles and conglomerate lying above the Apalachicola group (Oligocene) in this well probably 

 indicate an erosion unconformity between the Apalachicola and the Miocene. 



Miocene rocks are exposed at Shell Bluff, on Chickasawhay River, near Merrill, Miss., the 

 type locality of L. C. Johnson's Pascagoula formation. G. C. Matson has discovered that the 

 Miocene along the Alabama-Mississippi line is separated by an erosion unconformity from the 

 underlying upper Oligocene. Across southern Mississippi the Miocene is represented by estuarine 

 deposits that have not yet been definitely discriminated from the underlying upper Oligocene 

 deposits, which are also estuarine and of similar lithology. 



PLIOCENE. 



The marine Pliocene of South Carolina is a southward extension of the PUocene of North 

 Carolina. The principal area lies along Waccamaw River, near Conway, in Horry County. 

 The material is yellow, brown, and gray shell marl. Sloan gives the thickness at the mouth 

 of Tilly's Lake as lOJ feet. 



The Charlton formation of Georgia and Florida, which outcrops along St. Marys River 

 from Stokes Ferry, 11 miles south of St. George, Charlton County, Ga., to Orange Bluff, 2 

 miles above Kings Ferry, Nassau County, Fla., is probably of Pliocene age. The formation is 

 composed of calcareous shell marl, sand, and clay, having a maximum exposed thickness above 

 low-water level of about 15 feet. 



The most extensive development of the marine Pliocene in the Coastal Plain area is in 

 Florida, where it is divided, largely because of its areal distribution, into two formations — ^the 

 Nashua marl, which occurs along St. Johns River below the town of Nashua, and the Caloosa- 

 hatchee marl, which takes its name from Caloosahatchee River. Matson and Clapp ^^"^ give 

 the following description of these formations: 



"The Nashua marl bears a strong lithologic resemblance to the Caloosahatchee marl. 

 There is the same alternation of sand beds with shell marl. The matrix of the Nashua marl, 

 while usually calcareous, is always more or less sandy and sometimes consists of nearly pure 

 sand. The shells are commonly weU preserved, though locally a marl consisting of broken 

 and eroded fragments of shells is not uncommon. However, the organic remains are so well 

 preserved that it is easy to obtain good collections of fossils from this formation. 



"The Nashua marl is much thinner than the Miocene strata. This fact, together with its 

 distribution beneath the lowlands near the coast, indicates that the Pliocene submergence was 

 less extensive than the Miocene; and the presence of shallow-water fossils shows that the Pliocene 

 sea did not attain any great depth over that part of the State where the marine beds are now 

 exposed. The Nashua marl is seldom more than 6 or 8 feet thick, but locally it attains a greater 

 thickness. A series of samples of sand and marl from a well at DeLand indicates that at that 

 locality this marl has a thickness of about 32 feet. 



" The Nashua marl occupies the St. Johns Valley, where it underlies a broad terrace bordering 

 the stream. It probably occurs beneath the plain east of St. Johns River, but the overlying 

 Pleistocene forms such a thick mantle that the Nashua inarl has no influence on the topography. 

 On the whole, this formation has little or no influence on the topography of the State. 



"The Caloosahatchee marl consists of a light-gray shell marl which is often interbedded 

 with nearly pure sand. The matrix is usually very calcareous, but locally it consists of sand 

 and even in the most calcareous portion sand is abundant. The shells are remarkable for their 

 excellent state of preservation and their abundance in certain layers makes it possible to secure 

 excellent collections. 



"It is difficult to form a correct estimate of the thickness of the Caloosahatchee marl, but 

 its maximum is probably about 25 feet. Single exposures seldom exceed 6 to 10 feet, and the 

 average thickness is probably less than 8 feet. On the whole the Caloosahatchee marl is probably 

 to be regarded as thin, though it may thicken considerably toward the central portion- of the 

 peninsula and toward the southern end of the State. 



