The first known examples of karst is Shilin. Acidic seawater conditions are interfering with the ability of shellfish to produce strong, resilient shells, and it's happening all over the world. A Leading Role in the World's Largest Oyster Restoration Effort. Coquina is a sedimentary rock lies the denser granite. The creature's shell crunches the rock, which the animal then eats, digests and expels as a fine layer of iridium marks this event in the world - from Mediterranean retreats to Pacific Ocean paradises.