136 THE FLOOR OF THE OCEAN 



waves on the grand scale, but we have no record of their vigor- 

 ous onslaught along the continental terrace between Vancouver 

 and southern California — another belt with many canyons and 

 furrows. 



Thus the adequacy of earthquake waves is doubted; first, 

 because their reflux currents lack sufficient erosional energy in 

 the lower half of the continental slope; second, because these 

 catastrophic waves are too rare in both time and space. 



5. We come now to what seems to be the best hypothesis 

 on which to base explanation of the submarine valleys. The 

 essential idea may be summarized as follows: The trenching is 

 referred to bottom streams of sea water containing mud in sus- 

 pension and therefore temporarily endowed with density 

 greater than that normal to the clean water overlying the re- 

 spective continental terraces. It is further supposed that the 

 conditions for the formation of such bottom currents were 

 specially developed at certain stages of the Glacial Period, 

 though they may now be intermittently operating in a few 

 regions where the offshore terraces are narrow. 12 



About 40,000 years ago, the last set of Pleistocene ice-caps 

 of North America and Europe were of maximum total volume, 

 but were just beginning to melt away. With the exception of 

 a few small patches, the last remnants of these gigantic masses 

 of ice had disappeared by the year 7000 b.c. Since the water 

 represented in the ice-caps was evaporated from the ocean and 

 then dropped, as snow and rime, on the lands, the sealevel was 

 lowered everywhere. As already noted, the lowering was about 

 100 meters or 330 feet in maximum. From the amount of work 

 done by the last set of ice-caps, it appears that sealevel was 

 nearly as low during a period of the order of 50,000 years. 

 During three long intervals of still earlier Pleistocene time, 

 ice-caps had slowly grown to comparable size and then melted 

 away more or less completely. The second glaciation seems to 



