20 REPORT OF INVESTIGATIONS No. 50
deposited. In south-central Orange County, formations of the Ocala Group are missing, but in the northeast part of the county near Bithlo the Ocala is about 125 feet thick. The contours on the top of the Eocene limestones in figure 8 show the eroded surface of the Ocala Group except where the Ocala is absent. In these areas the contours represent the top of the Avon Park.
The Hawthorn Formation of Miocene age (about 25 million years old) unconformably overlies the Ocala Group and, where the Ocala is missing, the Avon Park Limestone. The clayey sand of the Hawthorn Formation retards the vertical movement of water between the water-table aquifer and the underlying limestone of the Floridan aquifer. In most parts of the county the Hawthorn retards, to varying degrees, the downward seepage of the water from the water-table aquifer. In low lying parts of the county where the artesian head is above land surface the Hawthorn Formation retards the upward movement of water.
The lower part of the Hawthorn Formation usually contains more limestone than the upper part. The limestone sections usually contain much phosphorite and quartz sand and may grade into sandstone known locally as "salt and pepper rock." In the northwestern part of the county, the Hawthorn Formation has a higher percentage of limestone than in the southeastern part.
Orange County lies in the intermediate/zone between the limestone-clay type of Hawthorn in north-central Florida and the clay-sand type of Hawthorn in south-central and southern Florida.
In Orange County the contact between the Hawthorn Formation and the underlying Eocene limestone is usually quite distinct; but the contact with the overlying deposits is gradational. The top of the Hawthorn is usually placed at the first occurrence of appreciable quantities of phosphorite or where a distinct and persistent greenish color appears. The Hawthorn is thickest (about 300 feet) in the southeastern part of Orange County and thinnest (about 50 feet) in the northwestern part of the county.
Undifferentiated sediments above the Hawthorn Formation may include the Caloosahatchee Marl (which has been designated Upper Miocene, Pliocene or Pleistocene by various workers) 2; thick deposits of red clayey sand which occur near the surface in some areas in western Orange County; and marine terrace deposits. The red clayey sand is used extensively in road building.
2-The U. S. Geological Survey gives its age as Pliocene.