NORTH AND SOUTH BRANCH 671 



obstructions at the foot of the declivity which caused most of it to turn 

 to the left through an angle of 80 degrees and to take a direction of 

 south 40 degrees. From the point where the turning begins at the bot- 

 tom of the steep declivity downward the rock stream features become 

 increasingly marked. At the upper end they are hardly noticeable. 

 From this point of first turning downward the rock stream undulates 

 with the course of the valley, gradually turning still more to the left 

 toward the bottom, till at the very end the direction of motion is such 

 that the stream from its source at the top of the ridge to the lower end of 

 the rock stream has turned through an angle of 110 degrees. Except 

 for the first great plunge, this great mass of rock detritus has evidently 

 flowed down the valley at a gentle slope of only 8 degrees, practically as 

 though it were a liquid. And yet there is no evidence of any appre- 

 ciable amount of fine material like mud that could have formed a lubri- 

 cant. Starting as it did, there could hardly have been much water 

 accompanying the rock debris, so that the moving mass must have con- 

 sisted essentially of rock debris and air. As in the case of the Elm and 

 Frank landslides mentioned above, the mass of rock fragments, moving 

 at great velocity, and with no lubricating material except air and a little 

 water, acts essentially like a liquid. In rubbing forcibly against each 

 other the fragments would naturally be broken finer and finer and be- 

 come more or less chipped. In fact, chipping of the larger fragments, 

 as though with a hammer, is quite a common feature. 



At the foot of the steep declivity where this north rock stream started, 

 the rock material is accumulated in a series of parallel running ridges 

 of considerable size. These ridges lie athwart the course of the descend- 

 ing landslide and are perhaps in part responsible for the defiecting of the 

 rock stream to the left. It would seem that the landslide, being sud- 

 denly checked at the bottom of the declivity, piled up in ridgelike rock 

 billows, and thus formed an obstruction that turned the course of the 

 rest of the stream into the side valley, down which it proceeded to flow 

 until its force was spent. In plate 48, figure 2, the scar left by the land- 

 slide at the head of this rock stream is very plainly shown to the left and 

 below the letter a. 



The South Branch 



The south branch is comparatively broad and short and does not pre- 

 sent such marked evidences of fiowage, except at the lower end, as is seen 

 in the north branch. The greater part of the area covered by this rock 

 stream is more of the nature of ordinary landslides, with hummocky 

 billows and hollows and a more or less level shelf marking the upper 



