18 McGee — Southern Extension of Ajypomattox Formation. 



but another phase of, the Port Hudson of Hilgard ; while in 

 western Mississippi the fluvial phase of the Columbia passes 

 into and is finally replaced by the vast body of loess with sub- 

 jacent gravel beds flanking the Mississippi river. These 

 various deposits are stratigraphically continuous, and form a 

 single and indivisible genetic unit; they were evidently laid 

 down during a single submergence of the southeastern coast, 

 extending from the terminal moraine at the mouth of the 

 Hudson to and beyond the Mississippi river ; and the local 

 variations in composition and structure are evidently due to- 

 simple and easily ascertained local conditions. 



The Columbia formation represents the first great episode 

 of cold in the Pleistocene ; and by reason of the recent work 

 on its southern extension, it is now possible to map with 

 approximate accuracy the geography of the southeastern part 

 of the continent during that episode. 



In its type locality the Potomac formation is a brackish water 

 littoral deposit made up of gravel and cobble-stones of quartz and 

 quartzite (derived respectively from the Blue Ridge and from the 

 veins intersecting the Piedmont gneisses), arkose (derived imme- 

 diately from the Piedmont schists and granites), sand, derived 

 from all these sources, and a considerable element of clay ; and its 

 age is probably early Cretaceous or late Jurassic — the abundant 

 plant remains indicating the former period, and the less abundant 

 vertebrate remains denoting the latter. South of the Appo- 

 mattox river the continuity of the terrane is broken by erosion, 

 and its surface is sometimes concealed by newer deposits ; but 

 exposures are sufficiently frequent to warrant the conclusion 

 that the Potomac formation is stratigraphically continuous with 

 the beds of gravel, arkose, sand, and clay exposed at many 

 points in the Carolinas and still better displayed at Augusta, 

 Macon, Columbus, and other points in Georgia, and so with 

 the body of like materials stretching from the Chattahoochee 

 to the Tombigbee in Alabama — the Tuscaloosa formation of 

 Smith and Johnson.* 



The Potomac formation is now connected, by actual observa- 

 tion of stratigraphic continuity between its most widely diverse 

 phases, and by identification at many intermediate points, from 

 its type locality on the Potomac to and beyond the Tuscaloosa 

 (Warrior). It is indeed variable in structure and materials in 

 different parts of its extent ; but the several variations are 

 easily traceable to local conditions of genesis. Viewed in the 

 large way, it is a. single and indivisible genetic unit, represent- 

 ing the first episode in the development of the Coastal Plain 

 of the Atlantic and the Gulf with its submarine extension — 

 the first episode in continental growth after the later throes of 



* Bulletin U. S. Geol. Survey, No. 43, 1887, p. 105. 



