A belt of beach rock surrounds the southern and northwestern sides of Heron Island. The rock is composed primarily of compressed algal and coral skeletons. Although the amount of exposed beach rock can vary from year to year, according the weather patterns and conditions, on the southern side it can be over 20 m wide. On this side, the beach rock can be divided into three bands, each with a different dominant invertebrate fauna. The upper zone consists of rock rubble and slabs that are inhabited by a variety of clustering gastropods, swift crabs, hermit crabs, anemones and amphipods. The middle band consists of hard rock an home to a large chiton, a number of gastropods and crabs. The lower, smooth region tends to be devoid of conspicuous invertebrates, although molluscs and crabs from the sandy zone can be observed in this area.