CONTINENTAL TERRACES AND SUBMARINE VALLEYS 151 



bottom current at Lake Mead, Dr. Kuenen deduced an ap- 

 proximate value for c that would match the conditions for 

 erosive furrowing of the continental slope. In the centimeter- 

 gram-second system of units the value for c came out at 400. 

 For our problem let us assume m to be 50 meters or 5000 

 centimeters (165 feet), a moderate estimate for the thickness 

 of silty water when transferred into a canyon by a first-class 

 storm. Let s, the mean slope, be taken at 1 in 15; and d at 

 only .0005. With corresponding substitutions in the formula, 

 the velocity is found to be about 160 centimeters per second or 

 5.75 kilometers per hour or 4 miles per hour — a velocity suffi- 

 cient to sweep along coarse gravel and to cause the tearing up 

 of silt and sand. Many silty currents of Glacial times must 

 have had initial effective densities higher than .0005; along the 

 shore-belts of the present day, which in general are less muddy, 

 the wave-stirred water has measured excess density at least 

 twice as great. 



The Kuenen experiments are particularly eloquent in show- 

 ing that, if the current running down the muddy continental 

 slope reaches a velocity of only two miles an hour, it should 

 take up a new, additional supply of mud and should, therefore, 

 run all the faster and erode still more efficiently. This new 

 power would, of course, be lost again when the current reaches 

 the lower, natter part of the continental slope. Kuenen's own 

 graphic account may be quoted: "The part played by the shelf 

 is now thought to be that of the ringing voice loosening an 

 avalanche. If the density of a comparatively small volume of 

 water is once raised above that of the deeper strata, the flow is 

 set off. It gathers volume and speed on the way down and 

 takes up more and more silt. Given a little time the canyon 

 erodes itself." This conclusion is all the more acceptable when 

 it is remembered that the terrace sediments are water-soaked, 

 hence highly mobile, and deposited near the angle of rest. 



