FLORIDA GEOLOGICAL SURVEY


water in the upper limestone. The water in the upper limestone becomes
salty downdip. Although few samples of water from these salt-water
zones are available for analysis, the zones of relatively fresh and salty
water may be distinguished on electric logs. An analysis of more than
60 electric logs was made for this purpose during the present study.
 Mineralization and hardness of ground water.-In addition to differ-
ences in salinity, ground water in the sand-and-gravel aquifer and in the
Floridan aquifer differs in amount of dissolved solids and hardness be-
cause of differences in lithology of the two aquifers. As might be ex-
pected, water in the Floridan aquifer (composed mostly of limestone)
is generally harder and more mineralized than water in the sand-and-
gravel aquifer, which is composed principally of relatively insoluble
quartz sand. As ground water percolates through the upper part of the
sand-and-gravel aquifer, it encounters very little soluble material and
remains soft and virtually unmineralized. However, harder and more
mineralized water comes from deeper wells in the sand-and-gravel aq-
uifer that penetrate sediments containing abundant sea shells. The
abundance of ground water remarkably low in mineral content has in-
fluenced several large industries to locate in Escambia and Santa Rosa
counties.
 Relation of quality of water to geologic history of the Gulf Coast.-
 For millions of years the Gulf coastal area has been slowly subsiding,
 forming a vast sinking trough, or geosyncline. As the trough sank,
 streams emptying into the Gulf of Mexico kept the trough nearly full
 by dumping into it huge quantities of mud, sand, and gravel. According
 to Howe (1936, p. 82), "These sediments have been concentrated along
 a narrow zone paralleling the present shore, and, since the beginning of
 the Eocene, have accumulated to a thickness which probably exceeds
 30,000 feet [south of the Mississippi River] the region of the pres-
 ent coastline has been depressed under the weight of these deposits to
 almost three times the present maximum depth of the Gulf of Mexico.
 The major axis of the Gulf Coast geosyncline approximately parallels
 the Louisiana coastline "
 Ground water in the Floridan aquifer in the Florida Peninsula be-
 comes mineralized as it moves through soluble limestones. In Escambia
 and Santa Rosa counties, however, these limestones have been depressed
 hundreds of feet by the sinking of the Gulf Coast geosyncline. This
 circumstance made it possible for rivers and streams to deposit the del-
 taic sand and gravel which make up the principal ground-water aquifer
 in westernmost Florida. The main area of subsidence did not extend far
 enough to the east to depress the limestones of peninsular Florida.