32 McGee — Southern Extension of Appomattox Formation, 



the Appomattox has heen largely removed by erosion ; on the 

 Oconee and Oo-eechee Rivers in eastern central Georgia the 

 monotonous plains formed by the coast sands of the Columbia 

 encroach upon and send tongues and fingers into the ravines 

 and broader depressions of a boldly sculptured upland of 

 Appomattox loam ; and in North Carolina and Virginia the 

 Columbia is little more than a flowing mantle masking the 

 more rugged framework of the older Appomattox. Indeed, 

 throughout their extent these formations illustrate the con- 

 trast between " topographic youth " aud " topographic old 

 age" as defined by Chamberlin — the one is soft faced, smooth, 

 nearly featureless ; the other hard-visaged, furrowed, strong 

 featured. 



Local unconformities between the Appomattox and the sev- 

 eral subjacent Neozoic formations are frequently exposed in 

 section; and general unconformity with all these formations 

 alike is indicated by its overlap upon all from the Grand Gulf 

 of the Miocene to the Potomac (Tuscaloosa) of the Cretaceous 

 or Jurassic. 



Especially significant is the unconformity between the 

 Appomattox and the Grand Gulf — the youngest of the series : 

 In southern Mississippi generally, and notably in the vicinity 

 of the Tallahoma river about Ellisville, there are sufficiently 

 numerous exposures of the siliceous clays constituting the 

 Grand Gulf to show that the surface of the terrane is one of 

 autogenetic sculpture, that the Appomattox was laid down as 

 a continuous mantle upon this sculptured surface, and that 

 after the close of the Appomattox period the rivers resumed 

 approximately their ancient courses and have impressed a new 

 and fairly consistent sculpture upon the old. So, while the 

 newer formation crowns eminences and floors depressions alike 

 where not profoundly eroded, its mass is little if any thicker 

 on the upland than in the valley, and exposures are as common 

 in the upper as in the lower slopes ; and along the larger rivers 

 the Appomattox has been frequently removed from the lower 

 slopes while it yet crowns the divides and highlands quite to 

 the brows of the bluffs. 



Especially significant, too, is the relation between the Appo- 

 mattox and the obdurate strata of the Choctaw buhrstone, 

 since a rough record of great continental oscillation is con- 

 tained therein : Southwest of Meridian and west of Corinne 

 lies a prominent ridge of the peculiar siliceous rocks of this 

 formation, making the divide between the Okatibbee and 

 Chunkee river. This divide is a meandering crest, sending 

 out lateral spurs and culminating in height at every bend, 

 separating a plexus of steep sided ravines, eoves and amphithe- 



