Sinkholes are formed by solution of limestone near the surface and consequent collapse of the overlying material into the solution cavities. If surface runoff carries clay sediments into the depression, a lake or marsh may form. Sinkholes are rare where the sedimentary cover over the limestone is more than about 60 meters thick. The physiography of Florida is the result of erosion, deposits, and solution-related processes that have formed the land over time. The sea island district of northeastern Florida ranges from upland terraces to plains and ridges.
Soils are dominantly sandy but significant expanses of clayey deposits exist.
The ecosystems associated with this district include pine flatwoods, dunes, and salt marshes. It originated as a series of barrier islands and lagoons and is presently consists of broad expanses of flatwoods with prairies, ridges, and a variety of coastal features. Soils are sandy with significant areas of peat deposits. The ecosystems associated with this district include pine flatwoods, prairies, cypress ponds, dunes, and mangroves. Parent material is largely limestone rock with a sand, marl, and organic material covering.