An atoll is a ring-like reef of low coral islands which enclose a central lagoon. The atoll is usually marked by isolated coral heads, and the central enclosed area is submerged, but covered with algal mud and coral sands.
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Atoll Glacier Formation Gilluly, J., A. C. Waters, and A. O. Woodford.
Principles of Geology.
W. H. Freeman and Company, San Francisco, 1975.
