Made from coral reefs and sand, Diego Garcia is one of these ring-shaped islands surrounding a lagoon
[DOC] [TLE] ATOLLS Definition Introduction - Springer [PAR] Atolls are ring-shaped reefs that occur in mid-ocean, often in linear ... oval-
shaped coral reef surrounding a lagoon in which there are no ... small sand cays 
(Shepard, 1948, p. 251).