154 THE FLOOR OF THE OCEAN 



The preferred explanation of canyon and furrow accounts 

 for another evident fact: a number of the canyons so far dis- 

 covered are en axe with master rivers respectively draining 

 adjacent continents. Examples are the intaglio trenches cut in 

 the submarine deltas of the Hudson, Congo, Niger, Indus, and 

 Ganges rivers. That the effect of each digging was at maxi- 

 mum would be a natural result of prolonged delta-building in 

 pre-Glacial time. For each of the deltas means a strong con- 

 centration of silt and therefore a special condition favoring the 

 development of silty bottom currents. 



Again, we note that, with the Glacial lowering of sealevel, 

 the master rivers of the continent were extended oceanward 

 and compelled to excavate "channels" across the continental 

 shelves. Some of them, with the anticipated maximum depth 

 of 300 feet, still exist in spite of the effort of wave and current 

 to fill the "channel" with sediment. That any of these channels 

 still remains open at all is testimony to the comparative recency 

 of origin. 



Finally, it may be remarked that emphasis on the special 

 conditions of the Glacial Period does not imply that some fur- 

 rowing of the continental slopes may not be continued to the 

 present day. Future investigations with the current-meter may 

 prove that silty currents, now being produced by great storms 

 and lashing their way across narrow continental shelves, may 

 run with eroding velocity. A case in point is represented in 

 Figure 82, which shows the continental terrace off California 

 to be only a few miles wide, and also portrays five canyons 

 that head close to shore. Here mud suspensions have to travel 

 only a short distance before they begin to rush down the steep 

 canyon floors, and can thus keep their individuality and per- 

 haps even pick up new, accelerating load of sediment. Thus, 

 along exceptionally narrow terraces submarine valleys may 



