132 FLORIDA GEOLOGICAL SURVEY-NINTH ANNUAL REPORT. 
ences in topography. One may assume that east of the Ocklocknee 
River is a structural ridge which brings the limestone somewhat 
nearer the surface, and that it is the approach of the limestone to the 
surface, that accounts for the differences in the topography. There 
is at present, however, no sufficient proof that such is the case. 
Another explanation seems possible, and that is the nearer approach 
to the Gulf Coast east of the Ocklocknee. From the Gulf inland in 
this limestone country the land surface is being lowered by the 
combined influence of surface erosion and underground solution. 
For some miles inland from the Gulf the. land surface has been prac¬ 
tically leveled. Farther inland is a belt in which the land surface 
has been but partly lowered, the basins representing the areas that 
have approached base level. 
If the rock structure conformed to the present Gulf margin, the 
lake region belt or belt of present active erosion should parallel the 
coast. The coast line, however, from St. Marks bends abruptly to 
the southwest and thus falls back from the limestone rock sub¬ 
structure which extends northwest from Peninsular Florida. 
STREAM DEVELOPMENT. 
The stream basins as well as the lake basins have a progressive 
development. If it is true that the Alum Bluff formation, consist¬ 
ing of sands and clays, formerly extended as an uninterrupted and 
unbroken sheet across this whole area, of which apparently there is 
no reasonable doubt, it follows that the streams first developed 
were not streams flowing on or in the limestone, but on the contrary 
were surface streams flowing across the sand and clay deposits, and 
were similar perhaps to the surface streams of Gadsden County at 
the present time. At a later stage in development the stream, hav¬ 
ing lowered its channel in places to the limestone, becomes in part a 
subterranean stream, and most of the streams of this area at the 
present time are in part substerranean. At a late stage in develop¬ 
ment the streams will again be surface streams flowing through 
channels lying in the limestone. The lower courses of the Wakulla, 
St. Marks and Aucilla rivers have all reached this late stage of de¬ 
velopment in which the stream flows in the limestone. The middle 
part of the drainage system of the Wakulla and St. Marks rivers 
illustrate the development of the substerranean streams. In this 
stage natural bridges of limestone above the stream channel are 
