Document ID: chunk:federal_register_of_legislation:F2015L01389:body:0:p53
Version: federal_register_of_legislation:F2015L01389
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Character Range: 154313–156987

Rise (Jongsma 1976).
Sediments on the Cocos Rise are very thin (100 metres to 200 metres thick) and no mineralisation or accumulation of petroleum has occurred (Jongsma 1976). The thickness of the corals underlying Cocos is not known, but the dredging of basaltic rocks in local waters suggests that it is in the order of 500–1,000 metres (Bunce 1988).
The two atolls are connected by a narrow submarine bank at a depth of 700–800 metres (Gibson-Hill 1948). Approximately 100 kilometres south-west of the Cocos (Keeling) Islands lies the Umitaka-Mary or Muirfield Seamount, which rises to 16 metres below the surface of the ocean. Taken as a series from Umitaka-Mary in the south-west to Christmas Island in the north-east, these seamounts display a sequence in the evolution of atolls.
Geomorphology and topography
North Keeling Island is approximately 2 kilometres long and 1.3 kilometres wide, with an internal lagoon, and a terrestrial area of 1.2 square kilometres above high water mark. It is approximately shaped like a figure eight, with its long axis bearing slightly north-east. Like the southern atoll, it is a true coral island.
In form, the island is low and flat. The shore rises fairly steeply to a height of 3–5 metres, and from this peripheral ridge the ground slopes gently down to a large, shallow lagoon which occupies the greater part of the interior (see Map 3).
The composition of the island varies from sand to coral rubble. On the northern shore there is a broad, sandy beach. This continues along the western shore but with varying amounts of shingle. In profile, the sandy beach rises up to about 4 metres above mean sea level. The southern shore of the island is composed of a spectacular steep shingle beach, with a series of berms, or ridges. The entrance to the lagoon was previously located on the eastern side of the island. In 2005, the entrance to the lagoon closed as a result of natural forces of deposition. Much of the eastern shore is composed of a series of shingle berms; these are particularly well-developed along the south-eastern shore, south of the closed entrance to the lagoon, but also continue to the north.
There are also outcrops of coral conglomerate. A broad platform of conglomerate extends out over the reef flat at the eastern part of the island. Along much of the southern and eastern shore, the conglomerate outcrops occur at the foot of the beach but contain a series of parallel rubble ridges, that are dipped and stratified like beachrock.
The Cocos (Keeling) Islands were the only coral atolls that Charles Darwin visited in 1836 as he developed his well-known theory of coral atoll formation (Darwin 1842), in