REPORT OF INVESTIGATIONS No. 41


the ocean. Low swampy areas occur throughout each of these divisions but are more prevalent in the flat-woods forest.
   The sand hills in the northern part of the basin are erosional remnants of the higher marine terraces which were between 100 feet and 270 feet above the present sea level.
   The sinks and lakes occur in the section of the basin west of Econfina Creek where they have developed within the sand hills. This area is typified by irregular sand bills and numerous sink holes and sink-hole lakes. The sink holes range in diameter from a few feet to broad flat areas such as those in The Deadening lakes area (see p. 73). This physiographic division was developed by the solution of the underlying limestone and the subsequent collapse of the overlying material into the solution chambers. Most of the lakes have no surface outlets.
   The flat-woods forest is the largest physiographic division of the basin. It is slightly rolling to flat land lying on the terraces below an elevation of 70 feet. Most of this division is covered with pines except for a few small areas cleared for agriculture. The flat-woods forest is well drained except for some low areas around the bays on the 0 to 10 and 10 to 25 foot terraces. During rainy weather these low areas of the flat woods become quite wet. A few small perennial swamps occur at various locations throughout the flat-woods forest. The largest is Bearthick Swamp southeast of Youngstown which covers an area of about 2,000 acres (fig. 2).
   The fourth physiographic division occurs adjacent to the gulf coast and is characterized by beach dune deposits and wave-cut bluffs. The beach dune deposits are the youngest sediments in the basin and are the most rapidly changing physiographic feature.
   The surface materials in the basin, on which the physiographic features have developed, are generally very porous, permeable sands. The sands form the water-table aquifer which is thicker in the sand hills (80 to 100 feet) than in the lower elevations of the flat-woods forest (10 to 30 feet) and thickens again along the coast (65 to 140 feet). The sands are missing only in stream channels and in some parts of the broad depressions of the sinks and lakes division.
   The sands of the water-table aquifer cover a relatively impermeable layer of sandy clay and clayey shell material which forms an aquiclude (a formation that confines water to aquifers above and below it) between the water-table aquifer and the artesian aquifers below it, as shown in figure 3. This aquiclude is present throughout the basin except where it has been breached by a collapse into solution chambers or by erosion along Econfina Creek.