132 THE FLOOR OF THE OCEAN 



logues of a special class of land valleys, namely, those that have 

 originated in, as well as deepened by, "spring-sapping." 10 Illus- 

 trations of these are found among the high plateaus of the 

 southwestern United States. The plateaus are underlain by 

 alternating beds of porous and tight, permeable and imper- 

 meable, rock. The rain-water fallen on the high parts of one 

 of the plateaus sinks to one of the continuous, permeable beds, 

 along which the water creeps, down to a lower level where the 

 bed crops out at the surface. Such a porous, continuous bed 

 may be conveniently referred to under the technical name 

 "aquifer." 



At the lower, outcropping end of the aquifer the subterra- 

 nean stream issues with kinetic energy and also with power to 

 dissolve rock matter. There the relatively weak, porous rock 

 is eroded away, whereby the overlying, stronger beds of rock 

 are undermined. Ultimately the undermined rock is torn apart, 

 with sliding of great blocks into the new gulch. The debris of 

 the slides is further broken up by frost and other weathering 

 agents, and, both in solution and by mechanical washing, is 

 carried away. Thus the new valley is gradually lengthened by 

 "spring-sapping." Because the valley has been formed by 

 slumping at its upper end, its head is blunt in ground-plan; it 

 belongs in the class of "box-head" canyons or gorges. 



The speculation that spring-sapping is the chief mode of 

 formation of the submarine valleys was based on a number of 

 assumptions: (a) each continental terrace includes an adequate 

 number of continuous, porous beds or aquifers; (b) the aqui- 

 fers crop out on the continental slope at the appropriate levels ; 

 and (c) there has been sufficent flow of ground-water (down- 

 creeping rain-water) along the aquifers to produce trenches of 

 great length and of great depth below their respective rims. 

 All three assumptions may well be doubted. 



While here and there permeable beds have been demon- 



