Geomorphology and TopographyGeomorphology and Topography

Geomorphology and Topography

North Keeling Island is approximately 2.0 kilometres long and 1.3 kilometres wide, with an internal lagoon, and a terrestrial area of 1.2 square kilometres above high water mark. Its shape is approximately rectangular 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, sandy-bottomed lagoon which occupies the greater part of the interior [Figure 4].

The composition of the island varies from sand to 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. Much of the eastern shore is composed of a series of shingle berms; these are particularly well-developed just south of the channel into the lagoon, but continue to the north as well.

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, almost closing the channel into the lagoon. 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.

There has been considerable speculation as to how North Keeling Island developed. The location of the remaining entrance to the lagoon is unusual because it is on the most windward side of the island, rather than in the shelter that might normally develop on the leeward. Indeed, the island is the inverse of the horseshoe shape that Guppy (1889) considered the typical style of development on the main atoll. This has led a number of observers, starting with Fitzroy (1839), to suggest that the island developed from a series of formerly unconnected islands.

Some indication of the age of the island is provided by research undertaken on the southern atoll. Samples of coral shingle from a trench through Home Island sediments, indicate an age range of 1,400-1,800 years B.P (Woodroffe et al. 1994).