2 4 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
' 1 
I 
drainage. Subdrainage is best developed in those areas where the 
underlying material is a porous limestone, allowing the water after 
working its way through the clayey sand, to escape into the lime¬ 
stone beneath. Under these conditions, sand may accumulate to 
any depth, depending upon the thickness of the original clayey sands 
and their permeability to water. Some of the deepest of these sands 
are found overlying limestones. In such locations, the clay con¬ 
stituent may largely disappear from the material leaving loamy 
sands. 
In considering the origin of the loose sands, the peculiarities of 
the parent formation, the clayey sands, must be borne in mind. A 
lirst effect of decay is the loss and obliteration of stratification lines, 
giving the material the massive appearance observed in all shallow 
exposures. Usually two or three well marked stages of decay may 
be recognized. The stratum of least decay usually seen at the base 
of shallow exposures, is mottled and blotched in appearance owing 
to the irregular depth to which decay has reached. The blotched 
areas follow the lines through which the surface waters have gained 
entrance, and as seen in cross sections, often show in vertical 
streaks or in patches. The patches and streaks are colored more 
intense red than the surrouuding sands, the development of the red 
color due to the oxidation of iron minerals being one of the early 
effects of decay. The thickness of this mottled stratum is variable, 
ten to twelve feet being often seen. The degree of mottling that is 
developed in an exposure depends largely upon the character of the 
material at that locality. A relatively high amount of clay in the 
sand favors mottling since the water permeates clay with difficulty, 
while nearly pure sands will scarcely become mottled at all owing to 
the fact that the water permeates them uniformly or nearly so. The 
Jine of of demarcation between the mottled clays and the material 
next above is often a well marked line and has much the appearance 
of a break in the formation. 
The stratum next above the mottled clay is usually a brick red 
loam. This is uniform in color as it has been thoroughly permeated 
by the surface waters and the iron minerals thoroughly oxidized. 
The clay minerals are well decomposed and the stratum has a loamy 
character although it still retains sufficient firmness to form vertical 
exposures. The amount of clay in this stratum may be consider 
able as it may have received more clay brought down by the perco 
lating waters from above than it has as yet lost to the underlying 
stratum. This is the material that has often been regarded as typi 
cal Lafayette. 
