CONTINENTAL TERRACES AND SUBMARINE VALLEYS I29 



oretically ascribed to subaerial erosion, that is, to ordinary 

 river-cutting. 



Fifty-five years ago F. A. Forel, the eminent Swiss hydrol- 

 ogist, considered an explanation that demands no temporary 

 emergence of the continental terraces, but assumes the canyons 

 to have been dug by bottom currents in the ocean itself. 8 

 Although Forel had little faith in his own idea, this in various 

 forms is now growing in favor among both geologists and 

 hydrologists. Among the reasons for the preference are the 

 weaknesses of all the alternative explanations yet published. 

 It will therefore be profitable to study the alternatives in order 

 and in some detail. 



1. The principle of local or regional uplift of the earth's 

 crust was already well established when Dana and Lindenkohl 

 wrote, and the idea of temporary super-elevation of the conti- 

 tiental terraces came easily to mind. But there are ample 

 grounds for doubting that this process has had anything to do 

 with the ruggedness of the continental slope. 



In the first place, geologists know of no case where regional 

 uplift of a mile or more has been soon followed by subsidence 

 of the same region and of the same order. In the present case 

 this kind of oscillation of the earth's crust is particularly in- 

 credible. The original formation of each continental terrace 

 and the grading of the continental shelf at its surface took 

 millions of years. During that long time, the crust was dis- 

 turbed so little that the outer limit of the continental shelf was, 

 in average, only 40 or 50 fathoms below sealevel. We know 

 also that at the present time the shelf is covered with water 

 which in average is not more than about ten fathoms deeper 

 than the water that lay on the shelf before the canyoning. In 

 other words, the uplift hypothesis demands that enormous 

 upheaval was soon followed by a sinking of almost exactly the 

 same amount. Such practically perfect reversal of movement 



