CONTINENTAL TERRACES AND SUBMARINE VALLEYS I43 



the whitish, silt-laden water of the river deployed on the lake 

 for some distance; he can also see that this water is sharply 

 bounded against the blue water of the lake. With simple appa- 

 ratus Forel proved the sharpness of contact to be due to an 

 almost vertical plunge of the silty water to the bottom of the 

 lake. He therewith showed that a silty current does for a time 

 preserve its individuality under the lake water. To what dis- 

 tance? Forel found an answer to this query also. The Swiss 

 hydrographers had made a second important discovery: the 

 surface of the sub-lacustrine delta of the Rhone is interrupted 

 by a channel-like furrow, extending six miles down the delta, 

 from the line where the silty water makes its initial plunge. 

 See Figure 73. This occurs just outside the jetties at the mouth 



FIGURE 73. MAP OF THE RHONE DELTA IN LAKE GENEVA. LAND OF THE 



CHABLAIS ALPS ( LOWER LEFT) SHOWN BY SHADING. AXIS OF TRENCH IN 



DELTA (CONTOURED) INDICATED BY THE BROKEN LINE. SUB-LACUSTRINE 



CONTOUR INTERVAL FIVE METERS. 



of the Rhone, seen in the southeast corner of the map. The 

 path of the bottom current is represented by the broken line. 

 The depth of the channel below its rim is considerable, with 



