PRESENT CHARACTERISTICS 491 



its topography the mass simulates a glacial moraine, for it is covered 

 with orderless humps and hollows, scattered ponds and marshes. At the 

 lower end the fnarginal slopes are relatively steep (plate 32) and rise 

 several feet above the adjacent surface. By obstructing transverse ra- 

 vines this steep edge has caused the production of small ponds. The 

 abundant crevasses constitute a novel feature of the surface not found in 

 moraines. These cracks will, of course, disappear in time, but now they 

 are conspicuous on every hand. 



Importance of Earth-flows 



Older earth-flows or landslides issue from most of the gulches along 

 the south slope of the Gros Ventre Valley and have been seen at other 

 points in adjacent valleys. At first glance they are easily mistaken for 

 glacial moraines, and where both occur together, as below Dorwin^s ranch 

 on the Gros Ventre, only the most painstaking and critical' study will 

 serve to discriminate the two types of deposits. 



In this region earth-flowage of the kind described is one of the most 

 important processes by which the adjacent mountains are wasting away. 

 It must be ranked with stream erosion, ordinary slumping and glacial 

 work, as an important means of getting material from the higher slopes 

 down into the bottoms of the valleys, where streams can continue tlie 

 deportation. That it is of great importance here and in a few other 

 localities, but negligible in most other mountain regions, I ascribe to the 

 fact that the operation of the process depends on somewhat unusual con- 

 ditions : very weak unctuous materials exposed along rather steep slopes, 

 and especially on slopes in which the strata dip downward with the sur- 

 face. It is well known that in most Kocky Mountain and other uplifts 

 the very weak strata, such as predominate in the Jurassic and Cretaceous 

 formations, have generally been planed off to lowlands, leaving only the 

 firm sandstones, limestones, and still harder beds to form the mountain 

 slopes. This is believed to explain the general absence of earth-flows 

 from such ranges as the Wind River Mountains, the Bighorn Mountains, 

 and many others of the Rocky Mountain chain. 



Classification 



1 find very few published descriptions of similar phenomena. In 

 Science^ Mr. Robert Anderson is reported as having described to the 

 Geological Society of Washington certain earth-flows near San Fran- 



2 Science, n. s., vol. 25, 1907, p. 769. 



