SECOND ANNUAL REPORT-TOPOGRAPHY AND DRAINAGE. 
39 
topography. Moreover, it is frequently difficult to determine whether 
shallow depressions are sink-holes or are merely due to irregularities 
in the surface of the sand. However, in the case of such deep de¬ 
pressions as the lake at De Funiak Springs, the sink-hole origin of 
the basin appears to be unquestionable. 
Sand Dunes: — Sand dunes and ridges are common, especially 
along the southern edge of the uplands, but they are seldom more than 
a few feet in height. Wind blown sands are probably much more 
widespread than is indicated by the surface topography. However, 
at the present time, the heavy precipitation, together with abundant 
vegetation, prevents the development of an extensive dune topography. 
COASTAL REGION. 
Streams and Ponds:—The coastal region of Florida comprises a 
belt of lowland which seldom rises above the 50-foot contour, and 
over large areas its surface is only a few feet above high tide. Its 
emergence from the sea took place after the drainage of the uplands 
had been well developed, and the rivers gradually extended their 
channels across it as new areas were added to the land. The Pleisto¬ 
cene sand which forms a large part of the surface has a gentle slope 
toward the sea and is occasionally crossed by small streams which 
flow in shallow valleys. Minor irregularities in the surface of the 
sand have given rise to shallow lakes and ponds which cover large 
areas during the rainy season. The difference in elevation between 
the bottoms of some of these ponds and the surrounding areas is fre¬ 
quently less than two feet. 
Ridges:—Scattered throughout the coastal region are small areas 
of higher land which in some places resemble sand ridges and in other 
places are very irregular in shape. In some places they are found to 
contain a core of rock which is covered by a thin mantle of sand, but 
frequently they appear to be entirely composed of sand. These areas 
represent the higher parts of the original sea floor, and their position 
was determined by the inequalities in the surface of the underlying 
rock or by unequal deposition of the sands. 
Terraces: — In the valley of the St. Johns River and at various 
points along the coast, the sands form a well defined terrace which 
rises twenty to thirty feet above tide. This terrace was observed at 
several localities and it appears to be the result of wave action during 
the Pleistocene. Similar terraces occur along the other streams of 
Florida and there is some evidence that there are others at higher 
levels; but their satisfactory discrimination will require a more de¬ 
tailed study than could be made in the time allotted to the field work. 
