120 FLORIDA GEOLOGICAL SURVEY-NINTH ANNUAL REPORT. 
LARGE BASINS WITH SHALLOW-WATER LAKES 
I AM ONI A BASIN. 
Iamonia basin lies near the north line of Leon County. The 
basin is irregular in outline, but has an average width of from one 
to one and one-half miles. The total length of the lake is from 
.twelve to thirteen miles. At its west end the basin connects with the 
swamp of the Ocklocknee River. During flood seasons the river 
overflows into the lake. Similarly a high stage in the lake results in 
an overflow into the river. Small tributary streams enter the lake 
from both the north and the south side, as well as from the east end. 
The tributaries are small flat-bottomed streams which are dry, ex¬ 
cept during the rainy season. The lake fluctuates according to the 
rainfall. The lake basin when full covers an area of about 6,500 
acres. Except at the west end, where it joins the Ocklocknee River, 
the basin is largely surrounded by the red clay hills characteristic of 
this part of the State. These hills rise to an elevation of from 50 
to 100 feet above the. level of the lake. 
The sink through which the water escapes from this lake is 
found on the north border. Limestone rock, probably of Upper 
Oligocene age, is exposed near the bottom of the. sink, the water 
escaping through or under these rocks. Above the limestone partly 
decayed sandy clays occur. These contain few fossils, although 
oyster shells were found in abundance at one locality. The total 
depth of the sink below the general level of the lake is not less than 
50 feet. The sink is formed, as is usual in this type of lake, facing 
■ an abrupt bluff 30 feet or more in height. A considerable, number 
of sinks are found around the border of the lake, especially in the 
vicinity of the one large sink which receives the drainage of the 
lake. The. formation of these sinks is doubtless due to the fact that 
the water entering the drainage sink spreads laterally in the under¬ 
lying limestone and dissolves the rock rapidly. The result is the 
formation by subsidence of numerous sinks adjacent to the drainage 
sink. The presence of these sinks also indicates the manner of 
enlargement of the lake basin, and indicates in each case the direc¬ 
tion of most rapid enlargement at the present time. At other times 
the enlargement by solution and subsidence may have been most 
active in some other locality or direction or part of the lake basin. 
This lake only occasionally goes entirely dry, and as a result a 
