Surrounding the southern part of Lake Okeechobee is the Fort Thompson Formation, made of dense, hard limestone, shells, and sand. Rain water is less likely to erode the limestone to form "solution holes"—smaller versions of sinkholes that do not intersect with the water table. In this formation the beds are generally impermeable. Underneath the metropolitan areas of Palm Beach County is the Anastasia Formation, composed of shelly limestone, coquina, and sand representing a former mangrove or salt marsh. The Anastasia Formation is much more permeable and filled with pocks and solution holes. The Fort Thompson and Anastasia Formations, and Miami Limestone and (x), were formed during the Sangamonian interglacial period.