126 THE FLOOR OF THE OCEAN 



geological time, and then, during post-Glacial time, was 

 stopped more or less completely. 



In general the canyons off other coasts have form and di- 

 mensions like those characterizing the canyons of the American 

 continental terrace. From these similarities we may well as- 

 sume that at least the majority of the canyons, the world over, 

 were simultaneously developed, as well as representing youth- 

 ful features of the submarine topography. 



Where mapped in detail, the submarine valleys seem to 

 differ significantly from the Grand Canyon of Arizona and 

 similarly walled, subaerial valleys exemplified in Figure 71. 

 The eroding river responsible for each of these land valleys has 

 cut down through rock layers of differing strength. Where the 

 hardest layers crop out in the valley walls, there are cliffs; 

 where softer layers crop out, there are much gentler slopes 

 made on aprons of fallen debris. More technically expressed, 

 the walls present cliff-and-talus topography. But, though indi- 

 vidual cases of similar variation of side slopes may be ulti- 

 mately found in the submarine valleys, no systematic alterna- 

 tion of cliff and talus is shown by any of the isobathic maps 

 now published. These maps suggest that the layered rocks 

 composing the great continental terraces are, with respect to 

 strength, relatively homogeneous. It is true that Dr. Stetson 

 has dredged occasional slabs of hard rock from the canyon 

 walls, while most of his samples from the walls were clays and 

 sandy clays, but there is no evidence that the harder beds sur- 

 pass a few feet in individual thicknesses or that their total 

 thickness represents an important fraction of the continental 

 terrace as a whole. 



Some of the canyons off California have walls of abnormal 

 steepness, even to approximate verticality. Near the heads of 

 these canyons Professor Shepard has dredged up pieces of 

 granite, torn from the walls, which he believes to be largely 



