GEOLOGIC CONDITIONS AFFECTING THE SLIDE 489 



The edges of these argillaceous beds are upturned along the crest of a 

 spur of the Gros Ventre Mountains, and in that situation readily become 

 soaked with water from rains and from the melting of the snow in 

 spring. At the lower edge of the slope the Gros Ventre Eiver is actively 

 widening and deepening its valley, and in so doing undermines the ad- 

 jacent slopes from time to time. These conditions together favor land- 

 slide action of one kind or another. What actually happened in the case 

 of the Gros Ventre slide may now be considered. 



History of the Slide 



Several years ago, before the disturbance began, the small valley now 

 occupied by the slide was bottomed with a hummocky sheet of old land- 

 slide material which originated before any of the present settlers came 

 to the valley. In that respect it resembled many other tributary valleys 

 in the vicinity. According to residents of the district, the slide first 

 came into action in May, 1908. So far as I am able to learn, no one 

 actually saw it begin; but it is believed by some that the initial move- 

 ment was fairly rapid if not indeed precipitate. When first observed, 

 the disturbance was manifested only at the head of the gulch, where large" 

 masses of the slippery Morrison and Sundance (Jurassic) clays had 

 slumped down along the steeper slopes, overturning trees and leaving a 

 general wreck. Either quickly or slowly, the impulse from this upper 

 mass was then communicated to the old landslide debris farther down 

 the valley, and that in turn began to press forward, bulge, and crack. 

 The novel thing about this case is that the movement of at least the 

 lower part was very slow and yet continuous, like that of a glacier. 



A man who passed along the valley of the Gros Ventre Eiver by the 

 main road in the fall of 1908 said that the jumbled mass of earth, rock, 

 overturned trees, and undergrowth could be plainly seen half a mile or 

 more above the road at that time, and I infer from his description that 

 it presented a steep outer slope not unlike that of a glacial moraine. At 

 that stage the only sign of disturbance at the road consisted of a long 

 crevasse on the eastern edge of what is now the slide, but that sufficed to 

 show that the lower mass was already beginning to move. Day by day 

 this large crack became wider and developed subsidiary fractures, but the 

 only sign of movement visible to the bystander was the constant falling 

 of small particles of dirt from the walls of the crack. 



Whether or not motion continued during the winter of 1908-1909 I 

 have not learned; but in the spring of 1909 the material in the lower 

 part of the valley slowly pushed forward and its surface bulged into low 



