GLACIAL-CONTROL THEORY OF CORAL REEFS. 199 



its present position. The East Indian shelves are so wide and so 

 protected from the waves of the open ocean, that they cannot have 

 been completely truncated by Pleistocene waves. The same is true 

 of the Alaskan shelf, which, however, was probably covered by a 

 frozen and largely waveless sea during the climaxes of glaciation. 



On the other hand, the more powerful sea waves can stir mud and 

 sand at depths of 35 to 50, perhaps 100, m.^^ From the known speeds 

 and depths of the currents in these seas, it is clear that mud and fine 

 sand can be transported in water as deep as 75 m.; in water still 

 deeper the average offshore current aggrades the sea floor only very 

 slowly. With a constant sea-level, therefore, the depth of water on 

 the broad, outer part of a continental shelf should be between 20 and 

 40 fathoms (35 and 75 m.). Usually the edge of the continental 

 shelf is stated to be at the 100-fathom or 200-meter isobath. This is 

 an error, for the charts of the world show the break of slope on the 

 shelves to be near the 40-fathom line. The study of the spectacular 

 Gulf Stream has doubtless helped to foster a wrong notion regarding 

 the power of more normal currents to stir and distribute bottom muds 

 and oozes. If mud once settles to the bottom, it takes a much stronger 

 current to stir it up again than to transport it in the suspended state. 

 In general, waves and currents together are competent to advance the 

 outer edge of a continental embankment with noteworthy speed, only 

 if the depth of water is there no more than 75 m. Bottom transporta- 

 tion in greater depths proceeds with exceeding slowness. 



The foregoing brief analysis indicates that large areas of the conti- 

 nental shelves, though not benched by the Pleistocene waves, should 

 now be covered with water of depth closely similar to the average 

 depth of the wave-cut Pleistocene benches on the oceanic volcanoes. 

 The discussion further indicates the very small ele\ation (a few meters) 

 which must have characterized broad stretches of the coastal plains 

 exposed by the Glacial diminution of sea water. The benching of 

 such areas, in case they faced the open ocean, would be rapid. 



Testimony of Islands Uplifted in Post-Pliocene Time. 



A second searching test of the Glacial-control theoiy may now be 

 reviewed. If it be correct, wide or narrow benches, at the depth of 

 75 m. or less, should be found along coasts which have suffered no 



38 O. Kriimmel, Handbuch der Ozeanographie, Stuttgart, Bd. 1, p. 112 

 (1911). 



