The rubble crest is part of the reef rock rim, which forms the highest part of the intertidal portion of the reef, usually a few centimetres higher that coral growth observed on the reef flat and slope. Breaches in this continuous rim around Heron Island Reef are formed by tidal channels, which flow heavily at low tide when oceanic levels drop below the water level on the reef flat. Coralline algae cement the upper surface of the rim, often mounting loose coral rubble and boulders to the solid reef. The rubble crest consists of both cemented and loose coral boulders and plates, some of which stand over 2 m high. This zone has a great diversity of mobile and sessile species, including a range of gastropods, brittle stars, crabs, shrimp, ascidian, sponges, bryozoans and polychaetes, most of which inhabit or take refuge under the boulders.
The reef crest. Photos by Sandie Degnan.