Aldabra is formed from late Quaternary raised reef limestones, averaging 2km in width and up to 8m above sea level, and rimming a shallow central lagoon. The limestone has been eroded over the years to form an dangerous terrain of sharp spiky rocks and numerous pits, making walking off established tracks unadvisable. Many of the pits contain fresh or brackish water that sits on top of surrounding seawater as a lens and rises and falls with the tides. Aldabra has monthly mean maximum (December) and minmum (August) temperatures of 31ºC and 22ºC respectively. Average rainfall, with Aldabra located in the relatively dry zone of the southwest Indian Ocean, is 1100mm per year. Climate is heavily influenced by the NW monsoon winds from November to March bringing the heaviest rainfall, with SE trades blowing throughout the remainder of the year. The lagoon at Aldabra is linked to the ocean by two major and one smaller channels and by several smaller reef passages. Tidal range is 2 to 3 m and results in large exchanges of water between the lagoon and open ocean through the channels. The main channel alone drains approximately 60% of the lagoon.

The scientific history of Aldabra encompasses almost 100 years of both terrestrial and marine based investigations. Early contributions regarding the flora and fauna, and indeed geomorphological structure, of Aldabra made it in 1910 one of the better known Indian Ocean reef islands. In the mid 1960s Aldabra was thrust into the international spotlight, being considered by the British Government as a possible air-staging outpost, with the threat of the construction of an airstrip and support facility.

Assomption

Assomption island is situated approximately 37km south of the eastern tip of Aldabra (9 43' S, 46 31' E). This raised coral island has a mostly gently shelving rocky coastline, with the exception of the western coast that has an almost uninterrupted sandy beach that extends for approximately five kilometres. There is a large shelf extending out to sea on the eastern side of the island and a very steep drop close to shore on the western side. Assomption is largely flat (see photo), and due to the devastating effect of guano mining which lasted until 1983, is dominated by expanses of bare rock and caves, and is sparsely covered with low. Two large sand dunes are prominent on the south eastern coast of the island, one of them reaching 32m high. There is a concrete runway that runs from between the two sand dunes on the south east to the permanently manned settlement which is situated on the more sheltered western side of the island. The settlement is surrounded by Casuarina trees and there is an abandoned coconut palm plantation just south of it.

Astove

Astove lies 1056km from Mahé and approximately 185km ESE of Aldabra (10 04' S, 47 44' E). Astove is a raised coral island with a substantial shallow lagoon (approximately 4.5 x 2 kilometres) opening to the sea through a single narrow channel on the south coast. The exposed eastern side of the island is dominated by low scrub and mangroves on the lagoon side, while the western side is quite highly vegetated. On the western side there is an abandoned copra plantation and an apparently healthy population of giant tortoises. Astove is no longer inhabited but it was once exploited for guano mining, and more recently for the production of cotton, sisal, maize and copra. There is an abandoned settlement on the western coast and a grass airstrip on the north east point of the island. Astove has a gently shelving rocky coastline to the east, an undercut coastline on the southwestern coast and a 3km long sand beach on the north west coast. The shelf slopes gently out to sea on the east side of Astove, but there is a dramatic cliff on the west side that drops sheer to about 100m and continues to drop steeply to over 2000m. The cliff starts directly at the end of the 300m wide tidal flat from barely 6m deep and has many caves.

St. Pierre

St. Pierre is approximately 704km from Mahé and 500km ENE from Aldabra (09 20' S, 50 44' E). St. Pierre is a small (approximately 1.2 x 2 km) raised coral island. Guano was mined here between 1906 and 1972 converting a once densely forested island of Pisonia, to the current barren pitted landscape. The western third of the island is now densely forested by Casuarina. St. Pierre is virtually inaccessible from the sea due to a steep undercut fossil coral shoreline that is only broken at one point. Here there is a small inlet (approx. 5m wide) to a sandy bottomed cove. There is a derelict jetty and settlement on the north west shore that would only be accessible by boat in the calmest of weather. Like all the other islands surveyed St. Pierre has a gently sloping seabed on the exposed south eastern coast and a steep drop off on the north west.