376 MRNARI) AM) l.ADI) [iCHAr. 15 



by fibrous algae. The tests of the Forauiinifera that hve on the flat are eventually 

 carried landward by waves to form the main constituent of the beaches that 

 fringe all atoll islands. Many reefs show a definite zonation parallel to the reef 

 front. Some of the zones are areas of rich coral growth. In the few places where 

 the reef jiavement has been drilled it is underlain by a solid plate of hard rock 

 10 ft or more in thickness, lying on unconsolidated reef sediments (Ladd and 

 Schlanger, 19(50). 



The living organisms of the reef margin are damaged by wave attack and 

 are the source of many of the sediments found upon the reef flat. Storm waves 

 quarry large blocks from the reef edge and cast them upon the flat where they 

 are later riddled by boring organisms and slowly broken up by wave action. 

 The sand and silt formed by wave erosion are worked across the flat to the 

 island beaches and may eventually be deposited in the lagoon. 



On the leeward side of an atoll the reef edge lacks the algal ridge, grooves, 

 buttresses and surge channels that characterize the windward reefs. Leeward 

 reefs may be broader than those of the windward side but they are not dis- 

 tinctly zoned. Passes through the reef are more commonly developed to leeward 

 than to windward. Most of these are shallow but in many atolls at least one pass 

 is as deep as the deeper parts of the lagoon. 



B. Islands 



Most atolls bear a series of islands spaced at irregular intervals along the 

 annular reef. The islands are formed on top of the reef platform. Most of 

 them are small and all of them are lo\^ . They are composed of reef detritus 

 piled up by currents, waves and winds. Except for layers of beach rock (con- 

 solidated conglomerate and sandstone) at intertidal levels, the material is 

 unconsolidated. It consists of sand, gravel and rubble from pebble to boulder 

 size, most of the coarser material being of coral origin. In the Marshall Islands 

 about 15% of the island beaches show exposures of beach rock. In most areas 

 the texture of the beach rock is similar to that of the material making up 

 the existing beach and the strike and dip of the beds of rock are nearly or quite 

 identical to those of the unconsolidated beach material, dipping away from 

 shore at about 9°. Islands situated on reefs that are subjected periodically to 

 heavy storms exhibit one or more ramparts built of blocks of reef rock and 

 coral heads. These structures apparently can be built, moved, or destroyed by 

 the waves of a single major storm. Inland from the beaches and ramparts, sand 

 and gravel flats occur and large areas are covered with coconut ]>alms, screw 

 pines and thickets of bay cedar and other plants. Low sand dunes may occur 

 inland from beaches. 



C. Lagoons 



Large lagoons with a fetch of 20 mi or more develop waves comparable to 

 those of the open sea and lagoonal reefs on the down-wind side take on some of 

 the characteristics of the seaward reefs on the up-Mind side. The prevailing 



