CONTINENTAL TERRACES AND SUBMARINE VALLEYS I4I 



positive and negative. We have already learned that the can- 

 yon floors off the eastern United States are veneered with fine 

 mud, deposited since the excavation of those canyons. And, by 

 hypothesis, that excavation was the product of a rhythmical 

 change of conditions, due to the alternation of Glacial and 

 Interglacial stages of the Pleistocene. 



Tenth: The deepening and widening of a subaerial gully 

 may be effectively stopped by climatic change, that is, by con- 

 tinued drought and absence of sufficient rainfall. The preferred 

 explanation of submarine canyon and furrow implies cessation, 

 or at least great weakening, of the submarine erosion when the 

 water which had been temporarily bound up in the ice-caps 

 was returned to the ocean. 



While emphasizing the many parallels between subaerial 

 and submarine gullying, it is well to remember a fundamental 

 difference between ordinary river and silty bottom current. 

 R. T. Knapp and H. S. Bell of the California Institute of Tech- 

 nology have recently described this contrast in graphic words: 

 In the case of the river "the water transports the sediment" of 

 the channel; in the case of the silty current "the sediment 

 transports the water." 



River water is about 800 times more dense than the over- 

 lying air. A thick silty current on the sea floor is likely to be 

 no more than one or two per cent denser than the overlying 

 clean water. These conditions for flow are quantitatively so 

 different that one might doubt that any silty current running 

 down a continental slope can have velocity enough to enforce 

 erosion. Such antecedent doubt is relieved when analogy with 

 another sort of bottom current in the ocean is recalled. This 

 second kind of current is also denser, "heavier," than the over- 

 lying water, not because of suspended silt but because the bot- 

 tom current is richer in dissolved salts than the water above. 

 Such so-called salinity currents were long ago demonstrated at 



