440 REPORT UNITED STATES GEOLOGICAL SURVEY. 



their volume they have fashioned typical terraces, which reach uj) the 

 larger valleys a greater or less distance, as noted in West Teton Yalley, 

 Good Fellow's, Fox, and West Pass Creeks. These terraces, in evety 

 respect typical examples of their kind, show different levels, 5 to 25 and 

 40 feet above the level of the streams, the lower, and of course most recent 

 ones often strewn with coarse materials which enter into the composition 

 of the deposits out of which they were moulded. Along the south and 

 west sides of the hasin the surface slopes down in a broad gentle terrace 

 to the lower level of the plain, in which the streams at their debouchures 

 have deepened their beds. These terraces seldom show marked or abrupt 

 borders along their lower edges, but instead they generally merge into 

 the lower level tract with but slight demarkation, and this generally to be 

 sought rather in the gravelly character of the soil, which usually marks 

 the edge of the terrace-border. 



The components of the general terrace formation that fills the basin 

 chiefly consists of abraded rock fi-agments, gravel, pebbles, and small 

 bowMers, intermingled with finer sediments, and very generally covered 

 with a mantle of fine light-drab or brown soil. However, frequent and 

 more or less extensive tracts are covered with gravel, like bars, where 

 the coating of finer soil has been washed away. In the lower portion of 

 the basin, the low banks along Pierre's Eiver reveal quite extensive ex- 

 posures of light earth, which recalls the modern or lake beds of Dr. Hay- 

 den, noticed elsewhere iu this region, underlying or incorporated with 

 the later volcanic flows. Their occurrence here, iu the near proximity 

 to the basaltic flows, strongly suggests the ascribed relationship. It 

 may be impossible to determine the vertical extent of these earlier 

 Quaternary deposits 5 but that they may reach a thickness of several 

 hundred feet in the middle of the basin depression and elsewhere, locally, 

 seems not improbable. 



Although, for all that c^n be said at this time, the superficial deposits 

 occurring iu the basin may not be treated strictly in accordance with 

 their chronological sequence, but, from the similarity of their compo- 

 nents, the fluviatile deposits may here receive brief notice. The posi- 

 tion of these deposits in the debouchures of the streams descending 

 from the mountains has already been referred to, as, also, their peculiar 

 conformation. It is rarely, so far as our observations went, that the 

 mouths of the debouching streams are filled with accumulations that 

 strongly suggest morainal origin. The barrier in the mouth of West 

 Teton Yalley and the ridges on the north and south may prove to be 

 the remains of the terminal and lateral morains of a glacier that once 

 filled that vaUey. But elsewhere, so far as observed, these deposits 

 have been remodeled to such a degree as to be clearly ascribed to the 

 work of the streams themselves. Their sources and the progress they 

 have made in the process of deepening their beds are all plainly re- 

 corded in the character of the deposits which they have buUt up along 

 the border of the basin. The short foreland streams south of West 

 Teton Eiver are i3aved with more or less rounded fragments of sand- 

 stone and volcanic rock, such as occur in situ in the great foreland in 

 this quarter ; while the larger streams have swept down immense quan- 

 tities of thoroughly rounded fragments of all the sedimentary rocks 

 which their courses traverse, and which, in Bear Creek, West Teton, 

 and Goodfellow's Creek, are intermingled with a large proportion of 

 metamorphic bowlders which were torn from ledges deep within the 

 range. West Pass Creek, and all the streams along the west side of 

 the basin, are strewn with sandstone and limestone bowlders only, save 

 occasional fragments of volcanic rocks 5 the absence of granitic rocks 



