200 
FLORIDA GEOLOGICAL SURVEY—I5TH ANNUAL REPORT 
ST. JOHNS COUNTY 
St. Johns County lies in northeastern Florida, between the St. 
Johns River and the Atlantic Ocean. It is underlain principally by un¬ 
differentiated sands and marls with a few interbedded clays of limited 
areal extent and thickness. 
Some good clays are found in the southwestern part of the county 
in the vicinity of Hastings. One of these occurs on the property of 
J. T. Minton, about one mile southeast of Hastings. This deposit con¬ 
sists of four feet of a gray clay overlain by one foot of soil. A marl un¬ 
derlies the clay. This deposit has been proven on forty acres at this 
place. 
This clay may be used for a good grade of common and face brick. 
It may equally well be used for common structural materials. It has the 
following physical properties: 
Physical Properties of Minton Clay, Hastings (Lab. No. 0 - 33 ). 
Plasticity, judged by feel. Excellent. 
Water of plasticity. 23.55% 
Pore water. 0.41% 
Shrinkage water . 23.14% 
Linear air shrinkage. 11.2 %. 
Volume fire shrinkage. 31.2 % 
Modulus of rupture, average. 837.7 pounds per square inch. 
Slaking test. 2 minutes. 
Steel hard at cone 1. 
Overfires at cone 5. 
Fire tests: 
Temperature. 
Linear Shr. 
Per Cent. 
Absorption. 
Per Cent. 
Porosity. 
Per Cent. 
Color. 
950°C. 
0.7 
20.40 
35.75 
Brick red. 
1050 
1.2 
11.40 
27.40 
Brick red. 
1150 
1.2 
10.80 
26.50 
Brick red. 
1190 
2.2 
8.55 
25.50 
Brick red. 
1230 
2.3 
8.30 
22.50 
Brick red. 
Sandy clays occur along the east bank of the St. Johns River. These 
are flood-plain clays of variable extent and thickness and are suitable 
only for an inferior grade of common brick. One of these clays ex¬ 
posed four miles east of Picolata on the St. Augustine road has the 
following section: 
Section of Clay Strata near Picolata. 
Feet. 
1 
10 
? 
Soil . 
Clay, red, sandy . 
Sand, grayish with clay 
