102 
FLORIDA STATE GEOLOGICAL, SURVEY. 
use the term Miocene in the sense in which it was.formerly employed, 
to include the Apalachicola group of the Oligocene. 
Chipola Marl MemberThe Chipola marl member forms the basal 
portion of the Alum Bluff formation and rests conformably upon 
either the Chattahoochee or the Hawthorne formation. At the type 
locality of the Alum Bluff formation it constitutes the basal division 
of that formation, but fa’rther north it thins and permits the sandy 
beds of the Alum Bluff to lap over on the Chattahoochee formation. 
This overlap is probably explained by the gradual shoaling of the 
water which prevented the development of abundant organic life to¬ 
ward the north. As the sands were deposited farther south, they 
gradually encroached upon the area where marl had previously been 
forming. According to this hypothesis, there would be no necessity 
for an erosion interval between the Chattahoochee and the Alum Bluff 
formations for a gradual emergence of the land or a filling of the 
shallow water along the margin of the sea would permit the deposition 
of sands upon the limestones and marls. 
When fresh, the Chipola marl member consists of a light gray to 
yellow marl, containing many shells and shell fragments. The matrix 
is composed of calcareous clay containing a small percentage of fine 
sand. When weathered, the marl becomes dark yellow or reddish- 
yellow from the presence of hydrated iron oxide. The character of 
the deposit indicates comparatively quiet water with conditions espe¬ 
cially favorable for the development of organic life. In some locali¬ 
ties this horizon is represented by a very sandy marl. The “Sop- 
choppy limestone,” which Dali has assigned to the Chipola marl hori¬ 
zon, varies from a soft white or light gray to a dense gray limestone, 
which is occasionally cherty. Some beds of soft sandstone are doubt¬ 
fully referred to this horizon. The beds contain layers which appear 
to have been broken and recemented, thus forming a brecciated lime¬ 
stone which is said to be somewhat phosphatic. Locally the rock is 
abundantly fossiliferous, containing imprints of shells and fragments 
of bones. 
The Chipola marl member of the Alum Bluff formation is not 
known to have a thickness of more than fifteen feet and the average 
is probably only a few feet. The limestone belonging to this member 
is so imperfectly known that it is difficult to make a satisfactory esti¬ 
mate of its thickness. The maximum exposure, which has been re¬ 
ported, is about ten feet. 
The fauna of this member has been studied by Dali, 1 who says: 
“The fauna comprises three hundred and thirty-three species,, to which 
we may expect additions on further exploration. A species of Or- 
1 Dali, Wm. H., Wag. Free Inst. Trans., vol. iii, pt. 6, 1903, pp. 1574-1575. 
