416 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



extensively removed, showing that the valley was probably excavated 

 subsequent to the flow, as there was observed no evidence of the volcanic 

 flow having filled an ancient valley-depression. On the contrary, these 

 deposits may be seen rising in the sides of the present valley, gradually 

 climbing to higher elevations until they reach the above-mentioned alti- 

 tude, when they cease. It is, however, quite probable that the West 

 Teton Valley was marked out at a time long anterior to the epoch of 

 volcanic eruption, but, as is the case with all the streams on this side of 

 the range, the channel Avas as yet much less profoundly excavated and 

 opened out in a broad shallow recess facing the basin. 



A remarkable feature of the range is its uniformity of outline, and 

 when it is observed that this uniformity holds no relation to the compar- 

 ative development and position of the rocky materials out of which it 

 was sculptured, it is difficult to conceive by what laws the erosive forces 

 were so evenly distributed as to produce the result embodied in this 

 almost rectangular mountain block, in one part entirely made up of met- 

 amorphic materials, in others largely moulded in sedimentary deposits ; 

 but the denuding forces have reduced the base to the even line traced 

 by the present western foot of the range, as though it had not to deal 

 with so diverse petrographical elements. The North Fork and Leigh's 

 Creek drainage have constituted the principal channels through which 

 this immense quantity of detrital material was transported and piled up 

 in a wide-spreading accumulation of tailings, to which is doubtless due 

 the low upland that dammed Pierre's Basin, and along which the North 

 Fork flowed, as upon a mole, and which was subsequently involved in 

 the volcanic flows which covered alike all the lower portions of the 

 country. There is scarcely room to doubt that this Archaean area w r as 

 the theatre of extensive glacial action ; but, unfortunately, time did not 

 permit sufficiently close examination to determine its character and 

 extent. 



The West Teton River has excavated the largest valley and canon 

 found on the west side of the range. The lower portion occupies a fine 

 valley half a mile to one mile in width, and for the most part it is filled with 

 level terraces of drift materials, clothed with herbage and margined along 

 the swift-flowing stream by beautiful groves of pine and spruce. Five 

 miles above the debouchure the stream branches, and thence their courses 

 are limited within narrow bounds, until they open out into the Alpine 

 basins which occupy so considerable areas in the heart of the range. 

 Entering the valley from the gentle sloping plain of Pierre's Basin, its 

 debouchure is partially filled with a low morain-like accumulation of 

 loose materials and fine soil, which is hollowed into w T ave-like inequali- 

 ties somewhat parallel with the foreland foot, and covered with clumps 

 of aspen. Within the valley huge blocks of limestone and granite bowl- 

 ders are strewn over the prairie surface and pave the adjacent acclivities, 

 erratics tumbled from the neighboring heights or transported by glacial 

 agencies from the interior portion of the mountains. One of the former, 

 a huge block of Magara limestone above 5,000 feet cubic contents, rests 

 upon the terrace level a mile or so within the valley, whose transporta- 

 tion from the nearest ledges of its kind could hardly have been effected 

 by other than glacial means. The volcanics show in rusty and brick-red 

 outcrops in the north side a couple of miles within the entrance, where 

 they hold a position several hundred feet above the valley-floor. There 

 appear the Carboniferous limestones, which are soon upraised sufficiently 

 to bring to view the Magara dolomitic limestones. The latter thence 

 form a line of picturesque castellated cliffs, gradually mounting and reach- 

 ing to the forks of the stream, in the vicinity of which they sweep round 



