SECOND ANNUAIy REPORT-SOUTHERN EEORIDA. 
231 
PLEISTOCENE AND RECENT HISTORY. 
While the limestone formations that have been described appar¬ 
ently extend under wide areas, they are, except the Key Largo lime¬ 
stone, relatively thin. They grow sandy toward the north and rest on 
less consolidated material or on formations containing sandy beds and 
are more or less covered by sand and marl. 
At some places along the east coast well records show a considerable 
thickness of coquina, and beds of well worn quartz sand and shell 
fragments; at others are relatively thin beds of limestone with sands 
and gravels above and below. Along the east coast and on the keys 
are rock outcrops fifteen feet or more above tide, along the west 
coast are none. The surface sands in some places are long, low ridges, 
in other mounds sixty feet high. These sand ridges often rise from 
ground where growth by wind action is now impossible. 
The events of Pleistocene time thus outlined include a period of 
submarine upbuilding of quartz sands and limy material moved south¬ 
ward by currents; a gentle depression of possibly 100 feet during 
which beach or bar deposits thickened on the east coast and the coral 
reef grew and spread south of the mainland, while behind the bars and 
the reef and on the west coast quartz sands and limy sands and muds 
accumulated in wide expanses of shallow water. 
Following this depression was a brief uplift of the land to possibly 
two hundred feet above its present level. Beach sands driven inland 
formed dunes. The limestones were eroded by the sea and honey¬ 
combed by the downward movement of surface waters. Following the 
uplift came a depression bringing the land to nearly its present level. 
While these are the broader features of the changes indicated in 
Pleistocene time it is possible that neither depression nor uplift were 
uniform, there may have been pauses -or even comparatively brief re¬ 
versals of swing; certain features of the keys and their shores suggest 
more than one elevation above sea level, but nowhere is there indication 
of a great uplift. In recent time the Everglades formed to the south of 
a lake larger than the present Lake Okeechobee. Sands and marls 
from the waste of the land and the ground-up remains of marine or¬ 
ganisms were deposited along the shores of the mainland and keys. 
Pine forests covered the sand hills and the topography of the mainland 
assumed its present aspect. Toward the Gulf Stream the present coral 
reef arose on the eroded surface of the old one. Waves and currents 
shaped marls and sands into the atoll-like group of the Marquesas 
while coarser materials were piled up to form the Tortugas. 
