CONTINENTAL TERRACES AND SUBMARINE VALLEYS \X] 



composed of such strong rock and therefore capable of being 

 precipitous. Besides having canyon walls of unusual steepness 

 and composition, the California terrace is exceptional in three 

 other respects. It is much narrower than most of the other con- 

 tinental terraces, is surfaced by hard rock in extraordinary de- 

 gree, and is situated in a region of recent dislocation of the 

 earth's crust on a large scale. Taking into account all available 

 information, it seems probable that at least some of the canyons 

 of? California were excavated above sealevel by ordinary rivers 

 and then submerged by sinking of the land. The preservation 

 of each in the form of an open trench, in spite of the tendency 

 of offshore sediment to fill it, can be explained by the same 

 kind of mechanism as that later suggested for the digging of 

 submarine canyons and furrows elsewhere. In any case the 

 nature and history of the continental terrace off California seem 

 to differ essentially from those of the terraces off the eastern 

 United States and other coasts. 



Origin of the Submarine Valleys. — Now for the genetic 

 problem. To guide us toward its solution let us review some of 

 the principal facts. First: The furrowing of the continental 

 slope is clearly the work of running water. Second : This slope 

 is steep, averaging about i in 15 and scores of times greater 

 than the channel of any long river on land. Third: Seismo- 

 graph and boring machine have proved the continental terrace 

 to be largely composed of water-soaked, poorly consolidated 

 clay and sand, offering little resistance to stream erosion. The 

 side slopes of most canyons and furrows are those expected if 

 they are underlain by this weak material. Fourth: The general 

 similarity of the canyons suggests simultaneous formation for 

 at least the majority. Fifth: Their distribution is world-wide, 

 planetary. Sixth: Judging by analogy with subaerial valleys, 

 nearly all of our canyons and furrows are to be rated as youth- 

 ful features of the earth's topography. The canyons off the 



