156 BALL AND STANLEY-BROWN APALACHICOLA RIVER GEOLOGY. 



The section was measured on the highest part of the bluff, which is 

 the first approached as the turn of the river is made in descending. 



Section at Rock Bluff. 



1. Superficial sands, thin and variable, say. . . 3 feet. 



2. Reddish clayey sand and gravel, about 15 " 



3. Greenish white compact marl, with fossils 67 " 



4. Chattahoochee hmestone, to water 12 " 



Total thickness above water 97 " 





-1 a V^(aI'S[ 



Figure 2.— Sketch-map of A him Bluff. 



Alum Bluff and Section. — Five miles below Rock bluff the reach termi- 

 nates by a semicircular bend convex toward the west and followed by 

 another convex to the east. The latter is bordered by a cliff called Alum 

 bluff, which is in township 1 north, range 8 west, section 24. It is hol- 

 lowed out of the fourth anticlinal fold south of the Florida line, extends 

 for about half a mile on the left bank of the river, roughly northwest 

 and southeast in trend, lower toward the north, highest in its south- 

 eastern third, and cut transversely somewhat southeast of its center by a 

 small deep valley of erosion. The bluff in a general way may be divided 

 vertically into two parts, the lower, forming a bench about midway, 

 crowned by a layer of acid clay from which water issues, leaving a saline 

 efflorescence on the face of the bluff, from which the latter derives its 

 name. Standing at the foot of the bluff, nothing can be seen above this 

 clay, the more arenaceous strata above having weathered back at a 

 lower angle, though still nearly vertical above. Upstream above the 

 bluff the river bank is a swamp, mostly of alluvial clay ; the right bank 



