30 



Plain in a belt four or five miles wide, sometimes swelling to a 

 width of ten miles, extending from Augusta through Aiken, south of 

 Lexington, through Columbia and on into North Carolina, through 

 Camden and Cheraw. The lower strata of the formation differ 

 markedly from the upper; they consist of coarse sands mainly with 

 pebbles, sometimes consolidated to a soft sandstone toward the south. 

 The upper beds are composed of finer sands and clays in the main. 

 However, there seems to be an intergrading of these two series of 

 strata, clays occurring in the lower and cross-bedded standstones 

 in the upper in places. The Potomac, outcropping here and over- 

 lying the Piedmont crystallines, extends on out beneath the later 

 formations, and has been reached and bored through or into, in 

 boring artesian wells at Florence, Darlington, Marion, Charleston 

 and other places. The beds in these places show a combined thick- 

 ness of 400 feet or more. Its slope toward the sea is greater than 

 the surface slope today. It is highly probable that there is a marine 

 phase of the Potomac into which the true Potomac, as just described, 

 intergrades. But the fossils reported by Tourney, as occurring in 

 these beds at the outcrop, are perhaps nothing more than the im- 

 pressions of mica plates so curved as to appear very much like the 

 impressions of shells or rather fragments of shells. 



The Marine Cretaceous unconformably overlies the Potomac in 

 the northeastern part of the State. The materials are marls, sands, 

 and clays with characteristic fossils. The formation seems to thin 

 out before reaching the Santee and Wateree toward the south, and 

 is in most places overlaid by some one of the Tertiary formations. 

 Along the Waccamaw, it is plainly exposed in many places and is 

 overlaid by the Pliocene. On the Peedee and some of its tributaries 

 the Cretaceous is also exposed, and in some places overlaid by the 

 Pliocene and in others by the Miocene. There is probably an ex- 

 posure or two on Black River, a few miles below Kingstree. The 

 formation, which is near the surface in Horry, Marion, Darlington, 

 Florence. Williamsburg, and Georgetown counties, after seeming to 

 pinch out just east of the Wateree-Santee line, thickens enormously 

 toward the south, so that at Charleston it occupies the greater part 

 of the depth between 450 and 1,950 feet in the artesian wells. But 

 some of this thickness at Charleston and in the southern part of the 

 State may be the marine phase of the Potomac. The distinction the 

 writer would make between marine Cretaceous and the marine 

 phase of the Potomac is simply one in point of time of deposition — 

 the marine Cretaceous being put down upon the Potomac after an 



