Early voyagers to the Pacific were often struck by the fact that coral reefs frequently possessed a slightly raised rim, or crestal ridge, especially on the seaward side, which tended to protect the reef surface from the action of the surf and at low tide was often dry enough for investigators to walk on or paddle around freely. Charles Darwin (1842) observed that the rim was not built of coral but largely of encrusting calcareous algae such as Lithothamnion . In some areas, the rim builders are predominately Porolithon .
In microcosm, the rim is observed also around microatolls (q.v.), where in addition to algae, the rim may be built of encrusting gastropod tubes, Vermetus , the pelecypods Mytilus or Brachyodontes and so on.
If one breaks the rim apart, it is sometimes found to have a core of reefrock, either coral or, in noncoral areas, eolian calcarenite (see Figs. 1 and 2). The organic rim material evidently acts as a protective plaster. Small changes of sea level (± 3m over the...
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References
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Fairbridge, R.W. (1968). Algal rims, terraces and ledges . In: Geomorphology. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-31060-6_5
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