SECOND ANNUAL REPORT-SOUTHERN EEORIDA. 185 
Evidently, therefore, the dunes were formed during a time when 
conditions were different from those now prevailing, a time when the 
land stood higher and the climatic conditions favored sand drift. Ly¬ 
ing near the coast they are not directly related to the present shore 
line, but, as shown by the fringes of swamp and the off-lying keys, to 
another shore line now below sea level. The significance of these facts 
as bearing on the post-Tertiary history of southern Florida will be 
discussed later. 
ROLLING SAND PLAINS. 
This term is used to include those sandy stretches of the mainland 
having broad swales and low ridges. In the swales are shallow lakes 
or lagoons,, wet prairies or cypress swamps. On the east coast these 
sand plains form a belt, having a possible maximum width at any one 
point of six miles, and extending from the north side of Palm Beach 
County nearly to Miami River. Out of this belt rise most of the dune 
mounds and ridges. Inland the rolling sand plains merge insensibly 
into the monotonous level of the flat lands and the prairies bordering 
the Everglades, seaward they are bounded by swamps, or by open 
water. 
On the west coast south of the Caloosahatchee River rolling sand 
plains are of relatively slight importance, though under this head may 
be included the arable land, a succession of beach ridges, back of the 
present shore line at Cape Sable and the sandy keys often not pine 
clad fringing the coast from Cape Romano northward. 
Near the shore on the east coast the higher ground and the ridges 
of the sand plains are frequently covered with a straggling growth of 
spruce pine. In the hollows are many fresh water lakes, some several 
miles long. Most of these are less than ten feet deep, and some are so 
shallow that they disappear entirely for months during such a period 
of deficient rainfall as prevailed from November, 1906, to May, 1908. 
A few of the lakes may be over ten feet deep and the writer was told 
that Lake Osborne had a maximum depth at the end of a normal rainy 
season of over twenty feet. However, the lakes as a rule are so shal¬ 
low and their banks have such gentle slopes that a survey of the rolling 
sand plain country made in or shortly after a summer of normal rain¬ 
fall would show vastly different relations of land and water than one 
made in early spring following a year of deficient precipitation. This 
accounts in part for lakes appearing on maps of southern Florida at 
places where the visitor may find none. 
The character of the sands, the elevation and prevailing trend, 
parallel to the coast, of the ridges, indicate that the rolling sand plains 
are largely the work of the wind and that they are related to the 
dunes. 
