Reefs are accumulations of organisms that form in warm, shallow ocean environments. Typically consisting of coral and algae, reefs are resistant ridges that rim islands, lagoons, and other shorelines (Figure ). Fringing reefs are flat expanses of reef that grow in the shallow water near the shore. Barrier reefs are elongate features that develop offshore parallel to the coastline and are separated from the coastline by deep lagoons. Atolls, circular reefs found in deeper water, are the result of reef development around the flank of a volcano that has since subsided but to which the corals are still anchored.