Seismic profiles from Lake Jessup show a high occurrence of subsidence. Areas of subsurface discontinuities predominate in two areas of the lake, around Caldwells Field and Bird Island (Index Map E). The area around Bird Island shows subsidence extending deep into the subsurface (A-A’ and B-B’), with a discontinuous strong reflection at about 16 m (52.5 ft, red). Gamma logs indicate the top of the Ocala Limestone to be at about 24 m (79 ft) below mean sea level, in close approximation to this reflection. Collapse in the Ocala results in subsequent subsidence in the shallower sediments, shown by the green reflections in example profiles. These sediments are the competent sands and clays of the Hawthorn Group. Accommodation-related stress fractures and slumping are also apparent around the areas of subsidence. Low-angle to horizontal reflections within the depressions (B-B’), along with a chaotic signal (A-A’), indicate differing processes of fill; with modes of transport ranging from fluvial to gravity (collapse) driven. The subsurface structure may be responsible for the presence and location of Bird Island. Profiles A-A’ and B-B’ show a structural high in the lake bottom created by adjacent areas of subsidence. This rise is translated to the surface where island development could have become pinned to this topographic high.

The area around Caldwells Field shows similar subsidence in the shallow subsurface, but collapse in the deeper reflection is not as apparent (C-C’). The Distribution of Features map shows the area where surface subsidence in this area occurs. The thickness of the overlying fill appears to be greater and is comprised predominantly of low-angle reflections. The fill appears to be more extensive than the underlying subsidence and is perhaps associated with the deeper areas of subsidence toward the central part of the lake.