McGee — Southern Extension of Appomattox Formation. 29 



The areal distribution of the remnants of the Appomattox 

 formation represented by present exposures cannot be set forth 

 in detail without large scale maps or more elaborate statement 

 than space will now permit ; but certain features of local dis- 

 tribution are too significant to be neglected. 



Throughout the Coastal Plain the formation is deeply dis- 

 sected if not completely divided by the larger rivers at and 

 commonly for long distances below its inland margin. The 

 tributaries have invaded it as well, and so too have the smaller 

 streams, down to the rivulet and storm-filled rill ; and thus its 

 entire surface has been sculptured by running water in a man- 

 ner well illustrating the type of configuration elsewhere 

 classed as autogenetic. Now many of the tributaries, as well 

 as some of the subordinate members of the wide branching- 

 drainage systems, have, like the principal rivers, cut com- 

 pletely through the formation and exposed the sub-terrane 

 over considerable areas ; and while the extent of the destruc- 

 tion of the formation in this manner is of course dependent 

 upon the local efficiency of the several factors of degradation 

 (declivity, stream-volume, texture of the rock mass, etc.), it is 

 evidently related in some degree to the character of the subter- 

 rane. This relation is well exemplified over the uplands 

 flanking the Tombigbee and Alabama rivers on the west. 

 Over the terrane of the Potomac formation the Appomat- 

 tox generally prevails, despite the considerable altitude and 

 high local relief, save in the valleys of the largest rivers ; over 

 the less elevated terrane of the Eutaw sands, it is more fre- 

 quently and more widely cleft by drainage ways, and its 

 remnants are thinner ; over the next newer formation (the 

 Tombigbee chalk) which lies low and flat, the greater part of 

 the Appomattox has been carried away, not only in the vicinity 

 of the Tombigbee river but all the way from northeastern 

 Mississippi to beyond the Alabama river, . so that it is com- 

 monly represented only by isolated belts and irregular patches 

 which, as Smith has shown, most frequently lie on northerly 

 slopes ; over the terrane of the Eufala sands, in which the 

 local relief again increases, the remnants of the Appomattox 

 quickly increase in number and expand until the formation 

 once more forms the prevailing surface on the uplands, though 

 the Cretaceous deposits are laid bare along most streams and 

 form the prevailing lowlands ; and over the eight or nine lower 

 Eocene formations into which the Lignitic of Hilgard has 

 been divided by Smith and Johnson, and among which clay is 

 the predominant material, the Appomattox still further expands 

 until it forms almost the entire surface, highland and lowland 

 alike, save in the valleys of the larger rivers. Still farther 

 southward lies the great siliceous deposit of the middle Eocene 



