74 FLORIDA GEOLOGICAL SURVEY-TWELFTH ANNUAL REPORT 
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mud in which the fresh water shells chiefly gastropods are im¬ 
bedded. This stratum has a thickness of three or four feet and is 
best seen underlying the basin of Lake Flirt from Old Fort Thomp¬ 
son to Coffee Mill Hammock, a distance of about eight miles. 
This marl is of fresh water origin, and may be quite recent in age. 
LIMESTONES IN LAKE OKEECHOBEE 
Within Lake Okeechobee there is apparently a reef of rock 
extending in a general northwest-southeast direction between 
Observation and Rita islands. At a point about 5 miles southeast 
of Observation Island the rock of this reef now stands above water 
at intervals for 1 a mile or so, the maximum exposure at the pre¬ 
sent low water stage being about two feet. At the surface this 
limestone is quite hard, or is streaked in a characteristic manner 
with alternate hard and soft layers. Beneath the surface, how¬ 
ever the rock is a rather soft oolitic marl or limestone of granular 
texture and light yellow color. The hard phase of this limestone , 
is much like the limestone found in the canal three miles west of 
Lake Hicpochee, while a thin stratum of a similar limestone is found 
near the surface at Coffee Mill Hammock. A few pieces of the 
marl phase of this limestone seem also to have been brought up 
from the lake at the entrance of the north New River canal. 
From the canals leading out of Lake Okeechobee to the south 
and southeast for a distance of about 25 miles very little rock has 
as yet been removed. Such fragments, as are seen along the canals, 
however, represent very hard compact fresh-water limestones. On 
the North New River canal dredging of the heavy limestone begins 
about 26 miles from Lake Okeechobee. The rock cut through on 
this part of the canal consists of a very hard compact close-grained 
limestone which breaks with a sharp fracture and will evidently 
make valuable concrete material. The same limestone is cut into 
on the south canal at 24 miles from the Lake. The very hard 
phase of this rock is a fresh-water limestone. As found on the 
banks of the canal, however, marine and fresh water limestones 
and marls are intermixed, indicating that there, as elswhere, the 
formation includes alternating fresh water and marine deposits. 
Pieces of this fresh water limestone are found on the North River 
canal as far as 42 miles from the Lake, although for the last three 
