Geography

It is located at 28°28′33"S 113°42′44"E, about 1.5 km (0.93205910497471 mi) east of West Wallabi Island

West Wallabi Island

West Wallabi Island is an island in the Wallabi Group of the Houtman Abrolhos, located in the Indian Ocean off the west coast of mainland Australia.-History:...

. Its nearest neighbour is Plover Island, about 700 m (2,297 ft) away. The island has an area of about 0.1 ha (0.247105163015276 acre), and a maximum elevation of 3 m (10 ft). It is uninhabitated, and devoid of human infrastructure.

This island should not be confused with the rocky island that lies to the west of North Island

North Island (Houtman Abrolhos)

North Island is the northernmost island in the Houtman Abrolhos, a coral reef archipelago in the Indian Ocean off the coast of Mid West Western Australia. Located about 14 km from the nearest island group, it is one of the largest islands in the Houtman Abrolhos, and one of the few to support...

. The latter has no gazetted name, but is informally known as "Shag Rock".

Geology and physiography

Shag Rock is essentially an outcrop of Wallabi Limestone

Wallabi Limestone

Wallabi Limestone is the name given to the dense calcretised, limestone platform that underlies the Wallabi Group of the Houtman Abrolhos, an archipelago off the coast of Western Australia. This platform, which arises abruptly from a flat shelf, is about 40 metres thick, and is of marine biogenic...

, a dense calcretised, coral

Coral

Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...

 limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 platform that underlies the entire Wallabi Group. This platform, which arises abruptly from a flat shelf, is about 40 m (131 ft) thick, and is of Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

 origin. Reef that formed during the Eemian Stage (about 125,000 years ago), when sea levels were higher than at present, are now emergent in places, and constitute the basement of the group's "central platform" islands, of which Shag Island is one.

Flora

About half of the island, including most of its centre, is rock with no vegetation cover. A very small area just south of the centre is vegetated by Tecticornia halocnemoides

Tecticornia halocnemoides

Tecticornia halocnemoides, commonly known as Shrubby Samphire or Grey Glasswort, is a species of succulent, salt tolerant plant endemic to Australia. It grows as a spreading or erect shrub up to fifty centimetres high...

(Shrubby Samphire). The remainder of the island, predominantly the northern third and a patch in the south west, is vegetated by Atriplex cinerea

Atriplex cinerea

Atriplex cinerea is a species of plant in the Chenopodiaceae family. It occurs in sheltered coastal areas and around salt lakes in the Australian states of Western Australia, South Australia, Tasmania, Victoria and New South Wales....

(Grey Saltbush), Nitraria billardierei

Nitraria billardierei

Nitraria billardierei is a perennial salt tolerant shrub. It is often found in saline areas or areas that have been overgrazed. Nitre Bush flowers mainly in spring with small ovoid or oblong fruit that are purple, red or golden...

(Nitre Bush) and Pittosporum phylliraeoides (Weeping Pittosporum).