214 



ELIOT BLACKWELDER 



Wind River, and Owl Creek ranges. In the Teton Range, the 

 juxtaposition of very hard and very soft rocks along a fault has 

 created abnormally steep slopes,^ so that the Archean rocks of the 



' The magnificent east front of the Teton Range can scarcely fail to suggest to 

 the geologist a recent fault scarp, but the consideration of it from various angles in 

 three different seasons has convinced me that it is in fact a "fault-line scarp" (W. M. 

 Davis, Bull. Geol. Soc. Am., XXIV [1913], 187-216) situated along a Middle Tertiary 

 displacement, of which the original topography has long since been destroyed. The 

 facts which suggest a recent fault scarp are the abruptness and continuity of the wall, 

 the comparative straightness of the base, and the triangular facets at the distal ends 

 of the very short steep spurs. The gravel-strewn floor of Jackson Hole seems also 



F'lioceneiV Surfac 



Fig. 27. — Diagrams of the Teton front to explain two possible origins of the 

 escarpment: {a) as a modern normal fault scarp; {b) as a fault line scarp discovered 

 entirely by erosion. 



to indicate the thick alluvial deposits normally made on the downthrown sides of new 

 faults. On further consideration, however, it appears that all of these phenomena 

 may be satisfactorily explained without assuming a recent fault, and there are addi- 

 tional facts which indicate that the scarp was produced by the exhuming of a once- 

 buried fault surface. Such a scarp would be abrupt and continuous, and as straight as 

 the fault trace. In the early stages of dissecting the scarp, triangular facets would 

 be developed at the distal ends of the spurs. The alluvial filling of the Snake River 

 valley is of unknown depth, but the exposures of the underlying rock in several parts 

 of the floor of Jackson Hole suggest that there is only a veneer of alluvium, as in other 

 basins in Wyoming. That the gravel is somewhat thicker in Jackson Hole than else- 

 where may be due to the fact that glacial waters from the Jackson Lake moraine 

 poured a large volume of outwash southward along Snake River. (See Fig. 27). 



The facts which directly suggest that the fault is an old one, and the scarp due 

 merely to the great contrast in the resistance offered by the rock masses on the east 

 and west sides to erosion, are: (a) all other faults within 25 or more miles of this region 

 are old faults, which have lost all original topographic expression; {b) it is particularly 



