9 6 FLORIDA GEOLOGICAL SURVEY—‘NINTH ANNUAL REPORT. 
TOPOGRAPHY. 
The topographic forms of this area, the hills, valleys and lake 
basins seem on casual inspection to be irregularly placed and en¬ 
tirely lacking in system. That this is not true, however, becomes 
apparent upon close study of the region. Primarily the topographic 
forms, the hills and valleys, were determined by normal or usual 
surface streams, and to this extent fall into drainage systems with 
the usual highland divides and remnants between basins. In addi¬ 
tion, the land surface has been materially affected by underground 
solution in the limestone and subsidence of the overlying materials. 
This process of underground solution tends to interrupt the drain¬ 
age systems through the formation of isolated basins. Such basins 
may be formed in established drainage systems. In such cases the 
drainage may be in part, at least, reversed, the. water finding its 
escape into underlying limestones. 
The topography in this area is so directly dependent upon the 
geologic structure that an account of the topographic types may be 
deferred to follow a description of geology. 
GEOLOGY. 
The geologic formations found at the surface within this area 
are chiefly of Oligocene and Miocene age, although more recent 
deposits overlie these in places near the coast. The materials of 
these formations include limestone, clays, sands, sandy-clay and 
shell marl. The limestones are chiefly those of the Chattahoochee 
formation (Oligocene), while the. clays and sandy-clays are for the 
most part included within the Alum Bluff formation (Miocene). 
Most of the shell marls of this area, as well as some of the sands 
above them, represent the Choctawhatchee formation (Miocene). 
The following table presents a summary of the formations of 
this area, all of which are of Cenozoic age: 
Pleistocene. No marine fossiliferous Pleistocene known within the area. 
Pliocene. No marine fossiliferous Pliocene recognized. 
Upper Miocene. Choctawhatchee formation; shell marls and sand. 
Lower Miocene. Alum Bluff formation; calcareous sands and,clays. 
Oligocene. Chattahoochee formation; limestones and calcareous clays. 
Eocene. Not exposed at the surface, although reached in well drilling. 
