m Daly— The C oral-Reef Zone. ' 



in giving strength to the normal reef, the weakening of 

 Pleistocene reef-growth seems the more probable. How 

 extensively were the coral seas affected? In what con- 

 cerns the preparation of platforms for the existing reefs, 

 the complete answer to this question would be more vital 

 if there were not another efficient cause for the wholesale 

 inhibition of reef growth during Pleistocene glaciation — 

 a cause soon to be noted. 



The abstraction of water to form the ice-caps, together 

 with gravitative attraction exerted by those masses, 

 lowered the sea-level within the tropics. The amount of 

 the lowering was not equal for the different glacial stages 

 and, of course, varied from minimum (zero!) to mini- 

 mum (zero?) through a maximum, in each stage. The 

 lowering, like each succeeding rise, was slow — in each 

 case doubtless taking some thousands of years. The 

 greatest lowering of sea-level during the Glacial period 

 has been estimated at from 60 to 70 meters (33 to 38 

 fathoms). Several important effects should be observed. 



(1) Before the first shift of level, the water on the shelf 

 facing a fringing reef was normally so deep that the life 

 of the reef was threatened through smothering by shelf 

 sediment only at the rare intervals of exceptional storms. 

 During and after the lowering of sea-level, the shelf 

 water was withdrawn from the inner part of each shelf 

 and was shallowed ten to forty or more meters on the 

 outer part. The stirring of shelf muds and sands must 

 have been much stimulated. By reason of such turbidity 

 of the shelf water, if for no other reason, the fringing 

 reefs would not be continued as living ramparts, outward 

 and downward, as the sea-level fell. Likewise, coral 

 larvae, if present, could not successfully colonize the 

 shallowing shelf because of smothering. In other words, 

 the shift of sea-level, of itself alone, entailed a rarely 

 equalled destruction of corals and probably encrusting 

 Lithothamnia as well} 1 The combination of oceanic 

 chilling and smothering by sediment could hardly fail to 

 reduce coral life on most shores to a very low state, if 

 not to extinguish it altogether. 



(2) If protection by vigorous reefs were thus removed, 

 each new coastal plain must have been nipped by the 

 waves and the shore-line pushed landwards. However, 



11 In general, have down-ward, eustatic movements of sea-level, involving 

 turbidity of shore waters, been influential in the extinction of some coastal 

 species at intervals of geological time? 



