138 THE FLOOR OF THE OCEAN 



across the flat plain was not caused by the excess of density but 

 by unbalanced pressure in the atmosphere as a whole. On the 

 other hand, there can be little doubt that such a sandy cloud 

 would tend to sweep bodily down along any steep slope upon 

 which it might be pushed. 



The compulsion on the silt-laden water to dive and slide 

 along the sea bottom was, of course, lessened in proportion to 

 any settling-out of solid particles. However, such loss of excess 

 density took time. Observation shows that shore waters, agi- 

 tated by a storm, remain murky with sediment for many hours 

 after the storm has spent its fury. We may therefore assume 

 that a bottom current of Pleistocene time could run many 

 hours. Over the gently sloping shelf its velocity was com- 

 paratively small, and over this region some of the coarser 

 detritus must have settled out. But, during the long time 

 when sealevel was lowered nearly to maximum, the zone of 

 agitating waves was long situated at or close to the fall-off to 

 the continental slope. Without much delay the silty water 

 would then have been precipitated down the continental slope, 

 with a velocity much greater than any possible on the adjacent 

 shelf. With only moderate velocity the silty current would 

 cross the whole slope in some hours. If, during those hours, the 

 speed were enough to cause turbulent erosion of the slope sedi- 

 ment, the settling-out would be delayed and new solid particles 

 added to the soup-like suspension. Nowadays the zone of in- 

 tense agitation by the breakers is 50 to 150 miles from the outer 

 edge of the flat shelf; hence, now, during the long, slow journey 

 to the fall-off, a bottom current loses much of its operating load 

 by settling-out and cannot attain great speed down the conti- 

 nental slope. Is it not clear that the silty currents of each 

 Glacial stage should have been incomparably more energetic 

 on the slope than any current developed by storms in post- 

 Glacial time ? 



