CHARACTERISTICS OF THE NORTH ROCK STREAM 669 



8 degrees, at each side and at the end the mass of rock debris suddenly 

 pitches off at an angle of 35 degrees, so that the fragments are in a state 

 of unstable equilibrium and are easily set moving if one attempts to climb 

 over them. At the sides, therefore, there is a marked trough formed 

 between the rock stream and the rock formations in place^ a feature very 

 suggestive of the lateral moraines of alpine glaciers. 



The concentric billows and irregular hummocks that are so marked a 

 feature of many of the rock streams of the San Juan Mountains are not 

 very noticeable in this rock stream. In the central portion of the north 

 branch, where the stream suddenly changes to a much greater pitch, 

 there are several rolls or billows suggestive of the billows of a cataract 

 in a river. Three of these may be seen in plate 48, figure 2, at the lower 

 edge just to the left of the middle of the picture. The hollows or flats 

 are here indicated by the three parallel snow patches. In reality they 

 are much more pronounced than is indicated by the picture. In the 

 upper portions of rock streams, near the foot of the steeper part of the 

 landslide, hummocky billows and flats so characteristic of landslide areas 

 are much in evidence, but not so in the more characteristic rock stream 

 portions where the evidences of motion are most marked. 



Instead of hummocks and billows this stream is characterized by the 

 presence of parallel ridges and troughs that conform to the direction of 

 the rock stream and that persist for many hundreds of feet. The ridges 

 may be, and usually are, quite flat, while the troughs are much sharper 

 and narrower. By means of these troughs and ridges the direction of 

 movement may be traced as readily as m}^ means of medial moraines in 

 the case of actual glaciers. Plate 49, figure 1, shows this feature where 

 it is most pronounced at the junction of the two branches. Plate 49, 

 figure 2, shows the ridges somewhat flatter. This view is taken from 

 the south side of the stream below the junction of the two branches. It 

 shows practically the whole of the stream where it is narrowest, 500 feet, 

 and gives an idea of the uniform slope of the rock stream. The wooded 

 slope beyond the rock stream is composed of Carboniferous rocks. 



As will be seen in the description of the north branch, this rock stream 

 conforms, as do glaciers, to the larger irregularities of the valley, turning 

 with the valley; but, like glaciers again, it is unable to conform to the 

 lesser irregularities of the surface. A striking instance of this is seen 

 in the damming up, so to speak, of a flat side valley on the north side ol 

 the north branch several hundred feet above the junction of the two 

 branches. At this point the rock stream, like the lateral moraine of a 

 glacier, passes uninterruptedly across the mouth of this side valley at a 



