RATE OF MOVEMENT AND ORIGIN OF SLIDE 349 



The slide is covered with vegetation in various stages of destruction. 

 On longitudinal ridges small aspens seem nndistnrl^ed, but in most areas 

 the only plants that seem normal are small shrul)s. Dead trees up to 

 two feet in diameter are twisted, l)roken, uprooted, entangled. 



Rate of ]\Iovement of the Slide 



The lower end moves into tlie inner gorge of Bull Lake Creek, a stream 

 about 60 feet wide and very swift, and the stream is able to handle all of 

 the material brought in. A fairly well beaten path leads to tlie lower 

 edge from the foot of the mountain, l:)ut movement has prevented tlie 

 forming of a path on the slide itself. The writer has talked with only 

 one man, besides those in liis own parties, who has ever seen the slide, 

 and no direct observations of movement have been made. In company 

 with Prof. L. G. Westgate, the writer first visited it in August, 1913. 

 Photographs made in 1913, when compared with those made in 1916, 

 show material differences in the surface in various parts, but give no 

 basis for estimating the rate of movement. TJie above are the only data 

 available for estimating tlie rate of mo\'ement, but it must lie slow and 

 relatively constant. 



Origin of the Slide 



x4t the place of origin, of the slide the cliff on the north side is made 

 up of about 800 feet of limestone which rests on a shaly formation (Bull 

 Lake Creek beds) 300 to 400 feet thick. The shale is very slippery, its 

 crushing strength is small, and it appears that, unsupported on one side, 

 it is unable to uphold the vreight of the overlying rock and slides out- 

 ward, causing great masses of the cliff to topple over. This falls on the 

 shales below and accumulates to such depth as to cause movement in the 

 wet, slippery sliales which rest on the Deadwood sandstone or on other 

 slippery shales. At the place where the Bull Lake Creek shales no longer 

 form the bottom of the cliff the slide turns almost at right angles and 

 moves away from the cliff. 



Snow lies on the upper end of the slide until early in July ; the under- 

 lying Deadwood sandstone furnishes water to it all summer, and small 

 springs emerge on the surface in the dry est seasons, causing the shales to 

 be slippery during the entire year. 



Evidence that a vast amount of slumping occurs along the vertical 

 cliff, and that the cliff' is kept vertical by the slumping and movement of 

 the slide, is furnished liy the character of the vallev. Bull Lake Creek 



