CONTINENTAL TERRACES AND SUBMARINE VALLEYS II3 



is only a short distance from the fall-off to the continental 

 slope, but, according to the theory, the intense furrowing of 

 the flanks of the continents was due to the special conditions 

 of the Glacial Period, when there were drastic, world-wide 

 migrations of the zone of breakers out to the edge of the conti- 

 nental shelf and back again. 



Facts of Topography. — Proof that offshore waters hide long 

 and deep valleys came originally from fishermen, who, in 

 search of fishing banks, extended their soundings over the con- 

 tinental shelves of the world; from systematic soundings of 

 coastal belts as aids to navigation; and from soundings of 

 cable-laying companies, who needed to know the relief of the 

 sea bottom, especially where their cables were to come ashore. 

 The hydrographic surveys under government auspices were 

 naturally the most extensive. Early in the latter half of the 

 nineteenth century it became clear that the generally smooth 

 surfaces of the continental shelves flanking North America, 

 Europe, and Africa are interrupted by trenches or "channels," 

 each of which begins a few miles from the beach line and 

 continues in a fairly straight line across the shelf, all the way 

 to the fall-off where the continental slope begins. 



In several cases those shelf "channels," slowly deepening to 

 the fall-off, about 300 feet below sealevel, are continued into 

 the continental slopes. Here the widths of the valley-like 

 trenches are measurable in miles, while the floors lie thousands 

 of feet below the general surface of the continental slope. 

 Thus, within the belt of the continental slope, the compar- 

 atively narrow and shallowly incised "channel" of the shelf 

 passes into an intaglio trough with dimensions of the same 

 order as the Grand Canyon of Arizona. For the outer, deeper, 

 and wider part of each of these submarine troughs the name 

 "canyon" has been borrowed. Even by the older methods of 

 sounding it was shown that some of the "canyons" head far 



