GEOLOGICAL SUEVEY OF THE TERRITORIES. 145 



aiiTs was northwest. It leaves a wide belt of the sedimentary strata 

 on the east side, near Horse Plain Creek. At a point in the cafiou, 

 where Clark's Creek enters the Beaver Head from the east side, there 

 is an enormous belt of singular, slaty trachytes, forming high walls on 

 both sides of the road. Immense quantities of debris, composed of the 

 fragments of compact basalt, lie on the side and at the base of the hills 

 on either side. At the mouth of Horse Plain Creek the valley expands, 

 the Beaver Head Valley extending up to the southeast, reaching the 

 Eocky Mountain water- shed and Horse Plain Creek Valley trending to the 

 southwest, to the same great divide 5 both valleys are broad, fertile, and 

 are now occupied by settlers. The elevation is so great that the climate 

 is very severe during the winter. One mile below Beaver Head Caiion 

 the altitude is 4,988 feet; at the junction of Horse Plain Creek and 

 Beaver Head, nine miles above, 5,130 feet. From this point to the 

 main Eocky Mountain divide it is thirty-three miles, and the elevation 

 is 7,405 feet. 



Although the soil is fertile, and during the summer season the grass 

 is excellent, yet the altitude about tlie sources of these streams is too 

 great for successful farming or grazing. About six months of the year 

 the grazing is of superior character, but during the winter months I am 

 of the opinion that stock must be driven down below the caiion for 

 safety. At the junction of Horse Plain Creek with the Beaver Head, a 

 broad valley has been worn out of the uplifted ridges of Carboniferous 

 strata ; but just at the junction there is quite a conspicuous remnant of 

 a limestone ridge that escaped erosion, which forms a sort of land-mark. 

 On both sides of Horse Plain Creek, as well as the Beaver Head, the 

 Carboniferous beds are elevated in ridges inclining at various angles. 

 From its source to the junction of Horse Plain Creek, the Beaver Head 

 flows through a synclinal depression, the sedimentary rocks inclining 

 from the Black-tail Beer Eange on the east side, while on the west side 

 the same beds incline from a range that extends northward between 

 the Horse Plain and Beaver Head branches. Below the junction of 

 Horse Plain, the Beaver Head flows along a sort of monoclinal interval, 



^ while the Horse Plain, which comes in from the west, carves its valley 

 through the ridges nearly at right angles. At one locality, in an anti- 

 clinal valley, which runs up northward from Horse Plain Valley, the 

 quartzites and micaceous schists of the metamorphic group rise up 

 beneath the limestones and reddish quartzites of Carboniferous age, over 

 a small area. Thence westward we pass over ridge after ridge of lime- 

 stones, quartzites, and arenaceous clays to the sources of Horse Plain 

 Creek. Throughout all these valleys, and jutting up against the sides 

 of the mountain hills that inclose them on either side, the Pliocene 



■ deposits are always found of greater or less thickness. On the imme- 

 diate bottoms of the Horse Plain there is an unusual amount of the 

 alkaline efflorescence, or suljihate of soda, covering acres, as white as 

 snow. 



As we pass up the valley toward the divide, a great thickness of 

 sandstones and quartzites, at least 1,500 to 2,000 feet, is exposed above 

 the well-known Carboniferous limestones, forming ridges which rise 

 800 to 1,000 feet above the valleys. The quartzites are so compact and 

 durable that they do not disintegrate, and the hills as well as the val- 

 leys are covered with the stray fragments. Here and there a dark, 

 abrupt mass forms the summit of a hill, weathered, perhaps, into sharp 

 pinnacles, indicating a point of effusion of basalt. On a little branch of 

 the Horse Plain Creek, caUed by the Indians Sage Creek, there are three 

 10 G s 



