CONTINENTAL TERRACES AND SUBMARINE VALLEYS I3I 



sonic sounding, he would hardly have regarded his idea as 

 even a good working hypothesis. 



2. We now turn to Professor Shepard's explanation. Ac- 

 cording to him, canyon and furrow represent drowned valleys 

 which had been cut by rivers of the ordinary type, during an 

 interval of time when the continental slopes were temporarily 

 emerged because of a world-wide or eustatic lowering of sea- 

 level. 9 He ascribed the negative shift of sealevel to the with- 

 drawal of water from the ocean in quantity sufficient to build 

 the ice-caps of the Glacial Period. Many glacialists have com- 

 puted the maximum for the negative shift so occasioned and 

 agree on a value of about ioo meters or little more than 300 

 feet. Shepard has tried to show that the shift was approxi- 

 mately 3000 feet or nine times as much. However, the large 

 value comes out after making impossible assumptions regard- 

 ing both areas and thicknesses of the ice-caps. A lowering of 

 100 meters well accounts for the "channels" cut across the con- 

 tinental shelves where the Hudson and other master rivers 

 were extended out to the new, lower shorelines ruling at the 

 peak of glaciation. But manifestly the actual eustatic shift 

 would not cause emergence of the continental slopes, whose 

 rugged topography is the problem at issue. In any case, the 

 furrowing between the isobathic lines of 3000 and 10,000 feet 

 are left unexplained, even if Shepard's estimate of the world- 

 wide lowering of sealevel could be admitted. 



3. Since neither crustal displacement nor eustatic shift of 

 sealevel are adequate conditions, we are left with the remaining 

 suggestion — that the dissection of the continental slopes was 

 the work of currents at the bottom of the ocean. Based on this 

 idea, three contrasted explanations of canyon and furrow have 

 been proposed. 



We note first that given by Professor D. W. Johnson in a 

 recent book. He supposed that canyon and furrow are homo- 



