The extreme southeast tip of Isla Carmen in the Gulf of California features a pristine, white-sand beach under protection within the Loreto Marine Park, Baja California Sur, Mexico. Sand deposited on this 150-m long beach is highly enriched (>80%) in coarse debris derived from wave-crushed rhodoliths (unattached coralline red algae). The Pleistocene limestone deposit in the foreground also is composed of massive rhodolith debris that includes rare whole rhodoliths (concentric growth of coralline red algae in spherical mode up to several centimeters in diameter). Both the limestone formation and the modern beach are underlain by basement rocks (Miocene andesite), which crop out as a protective headland at the far end of the beach. Living rhodoliths, naturally pink in color (Rhodophyta), form a large carbonate bank covering 194 hectares (480 acres) offshore this beach. See accompanying article by Rafael Riosmena-Rodríguez et al. in the special thematic section on rhodoliths (this issue). Photo taken at low tide. (Photograph and caption by Dr. Markes E. Johnson, Department of Geosciences, Williams College, Williamstown, Massachusetts, USA).