Daly — The Coral-Beef Zone. 147 



once breached, the waves and currents would begin to 

 attack the wide mud and "sand" deposits of the lagoon. 

 The resulting turbidity of the water overlying the outer 

 slopes would there injure the chances for successful reef 

 growth. In addition, the outer face of the reef would be 

 subject to invasions of sediment which was eroded by 

 rains falling on the emerged parts of the lagoon deposit, 

 or stirred up by lagoon waves, and finally washed out of 

 the now shallow lagoon through original scour-channels 

 crossing the reef. Hence, here also, smothering by sedi- 

 ment was liable to inhibit coral growth more or less per- 

 fectly, even if oceanic chilling did not. 



If fringing and barrier reef growth ruled in each inter- 

 glacial stage, the waves of each glacial stage had much 

 work to do before they could strongly cliff the rocks of 

 the original, pre-Glacial land. Possibly cores of pre- 

 Wisconsin atoll and barrier reefs still persist on some 

 banks, while other banks, thoroughly abraded during the 

 Wisconsin stage, bear only post- Wisconsin reefs. Exist- 

 ing lagoons would correspondingly vary in depth. How- 

 ever, the general similarity of pattern in the atoll and 

 barrier formations of the Pacific and Indian oceans 

 appears to demand that the open-ocean shelves were 

 extensively abraded during a late glacial stage as well as 

 during the first glacial stage. 



The last rise of sea-level and the beginning of the melt- 

 ing of the Late Wisconsin ice-sheet were nearly con- 

 temporaneous and probably occurred 20,000 to 50,000 

 years ago. Then began the atoll and barrier reefs which 

 are living because they were able to keep pace with the 

 rising water-level. Some banks were, however, colo- 

 nized too late or not adequately and have remained reef- 

 less to this day. Others, for special reasons, bear 

 reefs of relatively weak or intermittent growth; these 

 reefs are still not able to lift their crests to sea-level and 

 remain "drowned" at depths of ten to twenty meters 

 (roughly five to ten fathoms), for that is the range of 

 depths where the great waves are notably destructive. 



Some reefs, originally fringing, followed up the rising 

 water-level, making barriers backed by narrow lagoons. 

 Other fringing reefs have been planted since sea-level 

 reached its present position. Growing outward into 

 relatively shallow water, certain fringing reefs have 

 broadened more rapidly than most barrier and atoll 

 reefs, which commonly face very deep water. 



