GEOLOGY CBOCTAWHATCHEE AND APALACHICOLA RIVERS. 97 
and the shell marl is not exposed and in many cases probably lies be- 
low water level in the lake. 
As a result of these eroding agencies the escarpment which 
divides the low land at the north from the higher land at the south 
slowly migrates southward. The country near the north margin of 
the plateau which now contains the many lakes is a transition belt. 
The presence of the lakes is evidence of the beginnings of the 
disintegration of the materials overlying the. limestones. On 
the other hand, occasional hills stand out to the north of the main 
escarpment which are remnants of the former northward extension 
of the plateau. Among such hills within this area may be men- 
tioned Oak, Sexton and Rock hills. Most of these have persisted 
because they are more or less protected by a somewhat indurated 
phase of the sand and gravel beds which are general in their dis- 
tribution over this area. The belt of country known as Holmes 
Valley near Vernon in Washington County, as has previously been 
noted by the writers, is a belt of country transitional from the low 
lands to the plateau. A similar transitional belt lying immediately 
west of the Choctawhatchee River is known as Euchee valley.* 
SUMMARY. 
The data that has been presented has shown that Eocene and 
Oligocene limestones project into Florida in a general northeast- 
southwest direction through the central part of this area. The 
southernmost exposures of these limestones are not on the streams 
at either the east or the west sides of the area, but on the some- 
what higher lands near the central part. This fact indicates that 
contour lines on the top surface of the Eocene may be expected to 
curve farthest to the south near or somewhat west of the central 
part of this area. The line of the southernmost exposure of the 
Eocene-Lower Oligocene limestones as indicated on the sketch 
map, page go, is believed to approximate rather closely to the 
location of the 50-foot contour on the top surface of these lime- 
stones. From the. structurally high land the slope or dip in the 
formations is perhaps strongest to the east or southeast, and less 
rapid to the west. 
* Fla. Geol. Surv., 4th Ann. Rpt., p. 113, 1912. 
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