A Nearshore Rock Ledge

Aerial photographs show that a nearshore rock ledge borders the seaward side of the Florida Keys and forms a major regional shallow (0 to ~4 m) seabed structure (e.g., Figs. 5A and 5B). The ledge has not been mentioned in the literature other than by passing reference to its seaward scarp that marks the boundary between nearshore hardbottom communities and biota that inhabit the muddy grass-covered environment of Hawk Channel (Marszalek, 1977). Although presence of the ledge is an interesting and potentially significant finding, evidence is currently insufficient to derive solid conclusions about its history or to evaluate its implications. The paucity of data on such a potentially significant finding unfortunately puts discussion of what is known about the ledge beyond the scope of this paper. We mention its presence, however, because the ledge is clearly visible in several of the photographs used in this paper to illustrate other geomorphic trends discernible in the seabed floor.

Figure 5. Photographs taken in 1991 show sinuous intra-island tidal channels in areas of (A, top) Lower Matecumbe Key (middle Keys), and (B, bottom) Moser Channel (lower middle Keys; see Figs. 1A, 3A, and 3B for locations; Money Key, Molasses Keys, and Pigeon Key are represented by dots in Moser Channel in Fig. 1A). White dotted line marks seaward edge of nearshore rock ledge. Note tidal-delta sediments within channels or at channel edges. Compare solid area of seismic line 16 in B with surface and subsurface features in Figure 6C. [click on images for larger versions]