US Daly— The Coral-Reef Zone. 



Glacial Controls over Reef Growth. 



For the sake of clearness the chief Glacial controls 

 affecting the living coral reefs may be briefly reviewed. 

 They are three in number : 



1. Cooling of the tropical zone. 



2. Repeated lowering of sea-level, involving three 

 main consequences : a. Interference with reef growth 

 through the special stirring of bottom sediment; b. 

 Temporary location of the levels of marine abrasion and 

 deposition at depths 50 meters or more below the present 

 surface of the sea; c. Very great increase in the total 

 length of the edges of shelves and detached banks where 

 reef corals could take root (because of the temporarily 

 diminished depth of water on banks). 



3. Final rise of sea-level, lessening the turbidity of 

 the water on banks and thereby ensuring long life for the 

 reefs. 



The principles noted under 2c and 3 operate in favor 

 of the view that the present epoch is witnessing a quite 

 extraordinary profusion and prosperity of reefs, whether 

 fringing, barrier, or atoll. Because of Pleistocene 

 marine erosion, shelf waters just offshore are now, in 

 general, deeper and therefore purer than were pre-Gla- 

 cial shelf waters at the same distance from shore. 

 Equally important was the lowering of sea-level, because 

 it so greatly increased the total linear mileage of shelf 

 edges, where the water became shallow enough for the 

 planting of reefs and yet was relatively free from dele- 

 terious sediment in suspension. The increase in the 

 linear mileage of barrier reefs, compared with pre-Gla- 

 cial barriers, has tended to increase the total linear 

 mileage of fringing reefs also ; for the- barriers, now 

 damping the open-ocean waves, are protecting the shore- 

 waters of the great shelves from excessive turbidity to a 

 degree not possible if the barriers did not exist. Hence a 

 large percentage of the existing fringing reefs are 

 healthily growing inside offshore barrier reefs. 



Objections to the Glacial-control Theory. 



Many complications in the history of the modern reefs, 

 apart from Glacial controls, have been described or sug- 

 gested in the writer's 1915 paper, but some of them may 



