SOUTH PEAK RIDGE. 175 



From Western's pass to the north base of Buffalo Peaks, a distance of 

 about 10 miles, the upper valleys of the Little Platte and of Rough-and- 

 Tumbling Creek form a continuous line of depression parallel to this ridge. 

 These two streams bend to the eastward and flow together at the south- 

 ern end of the Weston's Peak ridge, where the anticlinal fold dies out as 

 the ridge disappears under the plain. It is here that the geological struct- 

 ure of the range changes from a double anticlinal to a single monoclinal 

 ridge, a change which is shown in the varying strikes and dips of the low 

 hills at the junction of these streams. Along upper Rough-and-Tumbling 

 Creek the Paleozoic beds all dip eastward in apparent conformity, though 

 with some variation of angle, and continue' their regular southeast strike, 

 not only close up to the base of the Buffalo Peaks mass, but apparently be- 

 yond it, without any sensible change of direction. It would appear, there- 

 fore, that the flows of andesitic lava, of which these peaks consist, have 

 been poured out through the upturned strata and spread out across their 

 edges, covering thus a geological horizon extending from the Archean up 

 to the Upper Carboniferous, or possibly the Trias, in marked contrast to 

 the manner in which the intrusive sheets of the earlier eruptives have been 

 formed. 1 



Western slopes. From Weston's gulch southward bey ond the limits of the 

 area mapped, the western slopes of the range are composed of Archean rocks, 

 among which granite is very prominent. There are doubtless many eruptive 

 dikes cutting through them in this area besides those of White Porphyry 

 at the mouth of Granite Creek, represented on the map, but time did not 

 admit of a sufficiently detailed examination to determine their outlines and 

 location. 



Weston's gulch, below the junction of its two heads or forks, which run 

 with the strike of the formations northeast and southwest, is a straight nar- 

 row gorge cut out of Archean granite and gneiss.. Its form suggests partial 

 glacier carving, but later erosion has removed all traces of moraine mate- 

 rial except a few erratics. Below this narrow gorge it passes into an open 

 country, occupied by partially eroded terraces of the Quaternary Lake 



'A more detailed description of the Buffalo Peak region will be found in Bulletin No. 1, United 

 States Geological Survey, Washington, 1883. 



