Aerial photograph of Great Barrier Reef

Wikipedia. Cropped by author

A reef is an area of dangerously low water (6 fathoms = 11m or less at high tide) that is located far enough from dry land that its presence is not obvious under conditions of low visibility. Reefs are a hazard to navigation and, when located off a beach, can seriously restrict access to the beach.

Of particular significance to the Pacific War were coral reefs. These are large masses of limestone produced by colonies of coral, a primitive form of animal life that lives attached to the reef and filters plankton from seawater. Coral thrives in very warm, shallow, clear water, and coral reefs are thus common just offshore of tropical islands. Atolls are a particular form of tropical island formed almost entirely of debris from coral reefs that has been piled up by the action of wind and waves.

Because coral cannot tolerate polluted or fresh water, coral reefs do not form where rivers or streams enter the ocean. This often means that navigable channels, called passes, form in an otherwise solid reef surrounding an island. When an island is eroded to sea level, as in the case of an atoll, the pass usually remains, marking where a river once existed.

The Solomon Islands and New Guinea are almost everywhere surrounded by reefs, which were generally very poorly charted at the time war broke out in the Pacific. Neither the Japanese nor the Allies were eager to risk major warships in the maze of reefs to the north and east of eastern New Guinea, which explains why almost no significant naval engagements took place here during the South Pacific campaign. By contrast, the waters around the Solomons rapidly deepen offshore. Reefs are only found close to land, where they are less of a hazard to navigation, and numerous naval engagments were fought here.

Reefs were also an obstacle to amphibious assault. The Tarawa invasion nearly ended in disaster when unexpectedly low tides left so little water over the offshore reef that landing craft were unable to cross and the troops were forced to wade hundreds of meters to shore. Later landing operations either landed where there was a sand beach unprotected by a reef, employed reef-crossing landing craft such as the LVT, or employed underwater demolition teams to destroy the reef with explosives.

Coral itself proved to be a surprisingly good construction material. Besides being abundant in many locations, crushed coral formed a hard, durable surface when rolled and wetted down, its chief weakness being that it became rather dusty when dry. This could be overcome either by frequently rewetting the coral or by oiling the coral surface. Bulk coral is actually stronger than concrete or ordinary limestone and was used extensively by the Japanese in field works.


"Handbook on Japanese Military Forces" (1944-9-15)

Rottman (2002)

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