. Atoll research bulletin. Coral reefs and islands; Marine biology; Marine sciences. OCEAN. Taken from Atoll Research Bulletin No. 59. Figure 2. Schematic Constitution of an Atoll. around much of the atoll (figure 3). Bordering the land on both seaward and lagoon sides are con- tinuous expanses of fringing reef, bare at low tide and covered with shallow water at high tide. These Tarawa reefs are often 400 to 700 yards wide on both the inner and outer sides of islands. It should be noted that no type of rock other than limestone is exposed in the Gilbert Islands. The form of the rock may vary,
. Atoll research bulletin. Coral reefs and islands; Marine biology; Marine sciences. OCEAN. Taken from Atoll Research Bulletin No. 59. Figure 2. Schematic Constitution of an Atoll. around much of the atoll (figure 3). Bordering the land on both seaward and lagoon sides are con- tinuous expanses of fringing reef, bare at low tide and covered with shallow water at high tide. These Tarawa reefs are often 400 to 700 yards wide on both the inner and outer sides of islands. It should be noted that no type of rock other than limestone is exposed in the Gilbert Islands. The form of the rock may vary, however, from solid skeletons of corals cemented with various lime- secreting algae into a solid limestone platform, to sands and gravels created by erosion, comminu- tion, and subsequent deposition of fragments of corals and algae. Although hills are completely absent and maximum elevations above sea level are on the order of 20 feet, there is nevertheless a certain irregularity in atoll terrain (figure 4). Near the water on the ocean side, there typically is found a beach ridge, with steeper slopes toward the sea and more gentle declivities inland toward the center of the island. Elevations in the interior may be as little as 2 to 5 feet, from which a gentle slope again rises to a somewhat higher crest in the vicinity of the lagoon beach. Gradients on the lagoon side are gentle, and the slight elevation above the interior of the island is hardly noticeable. Even the slope down to the edge of the water, the inner side of the lagoon fringing reef, is quite gentle. On the ocean sides of islands, beaches are moderately steep and are formed of fairly coarse gravel-sized fragments of corals thrown up by the waves. On the lagoon sides, beach materials are finer grained, often approaching a powdery sand in consistency. Where little wave action is found, as in Temaiku Bight within the southeast angle of the atoll, the sediments that accumulate are fine grained and form extensive flats
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