As the heavier materials from the mountains make their way through the Plateau and Piedmont in the streams, creeks and rivers of the water transport system silts and clay are picked up as well. By the time the heavier materials reach the coast they have become sand and settle just off shore, while the lighter silts and clays settle in the calmer waters behind the barrier islands to become the black anaerobic mud of the marshes. These marshes are some of the most productive acres on earth. They supply an enormous amount of nutrients, which make our waters rich in marine life. At the same time these nutrients make our water turbid. Frequently the visibility at Gray's Reef is poor due to the tremendous amounts of nutrients in the water and the huge volume of sediments that are being flushed from the mainland especially during periods of heavy rain. (photo: Gray's Reef NMS)