Figure 17. A pre-1990 aerial photo shows shallow, 1-m-high tidal bars and perpendicular sand waves near the west side of the Marquesas Keys in the Gulf of Mexico (from Shinn et al., 1990; Fig. 6A, Tile 11). Many of these sand waves are awash at spring low tide. Strong, reversing, north/south currents produce the sandbar orientations. Turtle sea grass (Thalassia testudinum) and various species of a calcareous green alga (Halimeda) populate the dark areas. Corals are absent. Distance across photo is approximately 2.5 km.