Sounds like something out of mythology, but Halimeda and Penicillus are not Greek gods, they are actually algae species, two of the most common in the Turks and Caicos. Using sunlight and chlorophyll they extract calcium carbonate from seawater. They grow rapidly but expire rapidly too, at which point the organic portion of the algae dissolves into crystals which are expelled onto the ocean floor. (I think that’s a visual to be filed for painting inspiration, crystals slowly meandering to the bottom of the sea). Halimeda segments make up a major component of sand, and Penicillus, particles of marl, the fine white mud covering the banks. The coral reefs surrounding the islands are also a calcium carbonate producer, simultaneously providing homes for many organisms that are also producing the shallow water carbonate; shellfish, sea urchins, and sponges. These results of sedimentation are how the Turks and Caicos Islands were formed, and are continuing to form today. As mountain peaks and most land masses are slowly shrinking from erosion, these islands are perpetually expanding.
This post is a summary of a portion of Brian Riggs article “Geology of the Turks and Caicos Islands,” chapter 2 in A History of the Turks and Caicos Islands, Ed. Dr. Carlton Mills