144 Harper: Explorations in Georgia 1903 



of granite outcrops in the Piedmont region. Some of them will 

 be discussed in the latter part of this paper. 



The ordinary phase of the Altamaha Grit is readily distin- 

 gulshed, as far away as it can be seen, from all other coastal plain 

 rocks by its surroundings and by the appearance of its weathered 

 surfaces, which are just the color of pine bark or perhaps a little . 

 darker. (A fresh surface is pale-yellow or often coarsely mottled 

 ■with red.) Other coastal plain rocks contain more or less calcium 

 carbonate and are surrounded by dense vegetation, but the Grit 

 crops out in broad daylight and is often visible half a mile away. 



The Altamaha Grit as such is not known outside of Georgia, 



* w 



though it underlies about one-fifth of the state or a third of the 

 coastal plain, and extends nearly to the Savannah River on the 

 northeast and the Chattahoochee on the southwest. The Grand 

 Gulf, * a formation believed to be of the same age, extends across 

 the lower part of Alabama, Mississippi, Louisiana and Texas, 

 but its actual continuity with our Grit has not yet been estab- 

 lished. 



The boundary between the rolling wire-grass region (Alta- 

 maha Grit) and the lime-sink region (Lower Oligocene) w^hich 

 adjoins it on the northwest in Georgia (and bears a similar rela- 

 tion to the Grand Gulf region in the states farther west), is one 

 of the most trenchant boundaries in the coastal plain, and, when 

 one becomes accustomed to it, is about as easily recognized as the 

 fall-line, f In 1903 I traced this boundary, by crossing it at 

 every convenient point, through the counties of Dodge, Wilcox, 

 Dooly, Worth, Mitchell and Decatun ;[; A striking feature of 

 this inland edge of the Altahama Grit country, all the way across 

 the state, especially toward the southwest, is that it is marked by 



* Named for Grand Gulf, a settlement on the Mississippi River in Claiborne 

 County, Mississippi, where this formation was first distinguished by B. L. C. Wailes 

 about fifty years ago. 



f My first impressions of the change in aspect of the country in passing from one 

 region into the other, traveling southward from Cordele in 1900, were described in 

 the Bulletin a few years ago (28 : 458. August, 1901). But my interpretation of 

 the geological significance of this change, based on the best information available at 

 that time, subserjuent research has shown to be incorrect. (See correction above.) 

 My remarks on Pinus pains tr is in that connection were also largely erroneous, 



X In the spring of 1904 I traced it farther east, through the counties of Laurens, 

 Emanuel and Screven, nearly to the Savannah River. 



