Great Barrier Reef

The tiny coral polyps began building their great Reef in the Miocene Epoch which began 23.7 million years ago and ended 5.3 million years ago. The continental shelf has subsided almost continually since the Miocene Epoch so the Reef has grown upward with the living additions to the Reef in the shallow, warm water near the surface; live coral cannot survive below a depth of about 25 fathoms (150 ft or 46 m) and also depend on the salt content in sea water. As the hydrocorals and polyps died and became cemented together by algae, the spaces between the skeletons were filled in by wave action that forced in other debris called infill to create a relatively solid mass at depth. The upper reaches of the Reef are more open and are riddled with grottoes, canyons, caves, holes bored by molluscs, and many other cavities that provide natural homes and breeding grounds for thousands of other species of sea life. The Great Barrier Reef is, in reality, a string of 2,900 reefs, cays, inlets, 900 islands, lagoons, and shoals, some with beaches of sand made of pulverized coral.