CONTINENTAL TERRACES AND SUBMARINE VALLEYS I39 



Quantitative testing of this Glacial-control hypothesis is not 

 easy. Essential facts are buried in both space and time — under 

 the ocean and under the obscuring blanket of post-Glacial and 

 Glacial time. Yet there are already in sight valuable tests 

 which, taken together, encourage faith in the root idea as the 

 most promising of all those reported in print. These tests are 

 based on illuminating analogies in Nature, on laboratory ex- 

 periments, and on an engineer's formula relating to the flow 

 of liquids under gravity. 



An obvious analogy is that of the ordinary river, which 

 also flows along the bottom of an ocean — an ocean of air. One 

 condition for its descent is its possession of density greater than 

 the density of the covering air. Similarly, the silty submarine 

 current runs down the continental slope because of density 

 greater than that of the covering clean water. The mechanical 

 likeness to the rain-made streamlets that gully steep subaerial 

 slopes is particularly striking. The items of this comparison 

 are worth noting. 



First: The gullying, subaerial stream is intermittent, with 

 full momentum at times of heavy rainfall, and feeble or non- 

 existent at other times; the submarine silty currents had full 

 momentum at times of strong agitation of shelf sediments by 

 wind waves and tidal waves, and were feeble or non-existent 

 at other times. 



Second: Because the initial slope is steep, the main subaerial 

 gullies and generating currents are directed, in roughly parallel 

 lines, down the slope. The prevailing "drainage" pattern is not 

 of the sprawled-out variety belonging to dissected plains of low 

 slope. So it is with the rugged topography of the relatively 

 steep continental slope. 



Third: The subaerial stream represents concentration of 

 rain-water along lines of initial depression across the escarp- 

 ment and in channels dug along those lines. By hypothesis, 



