Masthead Island - Geomorphology and Landscape

Geomorphology and Landscape

Beach rock is well developed along the southern beach and an occurrence of older beach rock is situated some distance from the beach on the north western corner of the cay. Vegetation is similar to that occurring on Heron island.

The vegetation and tree heights and trunk diameters can be used to interpret coral cay dynamics and thus the growth of the cay. It is postulated that the 'nucleus' old growth Pisonia grandis forest pattern (with trees up to 23m high and 2.4m trunk diameter) is in the central western part of this cay and is able to provide identification of the older forested sections of the island.

The Capricorn and Bunker Cays form part of a distinct geomorphic province at the southern end of the Great Barrier Reef. The cays and their reefs lie on the western marginal shelf, and are separated from the mainland by the Curtis Channel. The cays are not generally visible from the mainland, although Masthead Island may be viewed from Mount Larcom on a clear day.

Geologically the cays are young, having developed during the Holocene period, they are mostly around 5000 years old. The sea level was much lower during the last Ice Age (at the end of the Pleistocene period) and the coastal plane on which today’s reefs and cays developed was completely exposed. Early in the Holocene (around 10,000 years ago) the sea level began to rise, until it stabilised at its present level around 6000 years ago. Once the sea level stabilised, it was possible for reef flats to expand and provide potential sites for the formation of cays.

Masthead Island Reef is an Elongate Platform Reef.

The coral cay is a Vegetated sand cay The sand cay is located to the lee of their reef flat.