CONTINENTAL TERRACES AND SUBMARINE VALLEYS 137 



have lowered sealevel a little more than 330 feet. During tens 

 of millennia each set of ice-caps grew in bulk; during other tens 

 of millennia each set had maximum total volume; and during 

 still other tens of millennia each set was slowly melting. From 

 beginning to end of each of the four Glacial states — chaptered 

 as growth, culmination and waning — the wind waves and tidal 

 waves were breaking on the continental shelves, far from the 

 existing shorelines. Thus, for a time totalling more than a 

 quarter of a million years, the waves were pounding the old 

 embankments of clay, mud, and sand. Along the temporary, 

 slowly migrating shores, storm wave and turbulent tide were 

 muddied to a degree far beyond that represented in the 'long- 

 shore water of the present day. While so charged with par- 

 ticles of solid rock, the silty water was effectively denser than 

 clean sea water, and as a temporary suspension, sank bodily to 

 the bottom, that is, to the surface of the continental terrace. 

 There the weighted water flowed slowly down the gently 

 sloping continental shelf, to run much faster after it had passed 

 the fall-off at the top of the continental slope. Such accelerated 

 density-currents along the sea floor were, according to the 

 hypothesis now to be discussed, the chief excavators of our 

 submarine valleys. 



A bottom current of the kind and on the scale described is, 

 perhaps, not easily pictured. However, here as so often with 

 Nature's hidden processes, the imagination can be guided by 

 analogy. Figure 72 is a photograph of a sandstorm, sweeping 

 over North Khartoum in Nubia. We see how the suspension 

 composed of air and sand hugs the ground, keeping its indi- 

 viduality, as its sinister front moves on. Note, too, that this 

 layer, effectively more dense — heavier volume for volume — than 

 the clear air above and in front of it, keeps pushing its toes out 

 over the plain, as would be expected because of the superior 

 density of the sand-laden mass of air. Of course, the motion 



