McGee — Southern Extension of Appomattox Formation. 31 



Briefly, the hypsographic distribution of the Appomattox 

 formation is essentially identical with that of the general sur- 

 face of the Coastal Plain from the Potomac to the Pearl, save 

 that the formation extends a little farther inland than the 

 mass of the Neozoics, overlapping for a few miles of distance 

 and a few yards of altitude upon the Piedmont crystallines 

 within the fall-line. 



Stratigraphic Relations. 



In several exposures on the Appomattox river at and below 

 Petersburg, the fluvial phase of the Columbia formation (as 

 developed in the Middle Atlantic slope) rests unconformably 

 on the surface of the Appomattox, and a like relation to the 

 inter-fluvial phase is displayed in several railway cuttings 

 south of Petersburg. In the excellent section at Columbia 

 the coast sand phase of the Pleistocene formation rests uncon- 

 formably upon the Appomattox ; and at Lively, Alabama, the 

 " second bottom " phase of the newer formation overlaps 

 unconformably an eroded surface of the older one. From 

 these exposures in section the two formations are known to be 

 diverse in age. 



The unconformity between the Columbia and the Appo- 

 mattox becomes more striking when the relations of the two 

 formations to the larger rivers are considered : Every great 

 waterway traversing the Coastal Plain from the fall-line to the 

 shore of Ocean or Gulf has for scores of miles trenched the 

 Appomattox to its base and commonly cut far into older strata, 

 and the orange loams and sands are usually removed from the 

 bottom and half the sides of the trough whose axis is marked 

 by the waterway ; while the same rivers are flanked by ter- 

 raced belts of Columbia loam overlying the degraded edges of 

 the Appomattox and the older strata alike, and little invaded 

 by erosion (except on the Savannah and the Congaree) save 

 that of the river channel. It is true that the Chattahoochee, 

 Tuscaloosa or Warrior, and some other rivers are locally 

 flanked by terraces of Appomattox materials ; but these ter- 

 races appear to be the product of local wave work during the 

 Columbia submergence rather than of the rivers and waves of 

 the Appomattox period. 



Still more striking does the unconformity appear when the 

 general configuration of the two formations is compared : 

 About Grand Bay and St. Elmo in southwestern Alabama the 

 Columbia forms a smooth, monotonous, sensibly horizontal 

 plain, while the knolls and uplands of Appomattox protruding 

 through the flat-lying sands exhibit well developed autogenetic 

 sculpture ; over the smooth plains of the Tombigbee chalk the 

 Columbia deposits skirt the rivers in sharp-cut terraces, while 



