Algal Cup Reefs
Algal cup reefs (also known as "boilers") are restricted to the margins of the platform: they are most abundant along to the south shore but there are two major clusters around the northern rim of the platform, one, the northeast breakers and the other, unnamed, extending northward from North Rock.
In plan view cup reefs are circular, ellipsoidal, oblate
or cresectric. All the examples have an elevated rim that is awash at low
tide and surrounds a central depression, a micro-lagoon, usually only a
few meters deep but occasionally up to 5 m deep. In three dimensions the
reefs are cup or vase shaped with rough inward sloping sides. Near the
base of many cup reefs, there is sand-floored moat-like depression and
numerous cavities a meter or more in diameter extending from the moat into
the base of the reefs.