„.^,,,^.^n GEOLOGY— FEATURES OF THE MOUNTAIN BORDER. J 33 



abrupt southern point of one the ridges of outcropping rock soon bend 

 on the one side east of north, on the other to the west of north, inclos- 

 ing between them the higher ridge of archtean rock. The eastern side, 

 however, dips gently eastward, frequently not over 20°, seldom reaching 

 40°, while upon the west the same ridge often stands nearly or quite 

 vertical, and, following it northward, the drag downward soon becomes 

 quite sufficient to fault the bed also, while, farther on, the increasing 

 fault may not upturn the beds upon its west side at all. Farther north, 

 after passing beyond the limits of the sedimentary rocks, the fault be- 

 comes far more difficult to trace in the more structureless archsean rocks. 



The echelon folds and faults that occur within my district are shown 

 by the first nine sections of Plate II and the accompanying map. The 

 auticlinals are designated by the letters «, ft, c, d, e, the same fold always 

 having the same lett'cr attached. Folds a and c are the two principal 

 ones. 



Circling around the southern end of the high schist ridge «, section 1,* 

 and dipping away from it to the east, south, and west, are the edges of 

 the Triassic red-beds which once mantled over it, but which erosion has 

 removed and worn downward, leaving but two prominent ridges now 

 projecting above the surface. Two or three miles to the east rises the 

 abrupt west face of the hog-back ridge formed of the variegated Juras- 

 sic shales, capped by the hard sandstones of Cretaceous No. 1, all dipping 

 gently to the east. The latter sandstones are here formed of two main 

 layers, separated by about 100 feet of softer shales and sandstones. 



This double ridge trends from here nearly south w^ est to where the Big 

 Thompson and Dry Creeks join, when, turning very abruptly, it assumes 

 a northwest direction. At the point of turning the southward dip of 

 the lower sandstone of Xo. 1 is 20°, that of the upper being 50°. The 

 outer beds likewise swing around, but in curves of much larger radius, 

 though they are much obscured by the soil and the gravel terraces. In 

 its northwest course the double ridge stands vertical like two parallel 

 stone walls, for about two miles, when it quite abruptly disappears, cov- 

 ered either by the thick soil present or faulted below the surface. (See 

 sections 1 and 2.) 



Along the west side of the valley the same beds again outcrop in their 

 normal position upon the face of the granite ridge h, dipping east at a 

 gentle angle, and, with a nearly southward strike, they may be followed 

 nearly to the Little Thompson. When crossing Dry Creek No. 1 has 

 an east dip of 20°, that of the overlying beds of No. 2 and No. 3 in- 

 creasing to 35° and 50° within a mile to the east. 



Before reaching the Little Thompson, however. No. 1 again steepens 

 in dip, and the strike suddenly swings from soutb to west and northwest, 

 inclosing a low anticlinal hill (b, section 5) of Jurassic beds, on the south- 

 west side of which it appears standing nearly vertical. 



Following it north it shortly disappears, and, as the Cretaceous shales 

 now lie upon the west, both a fault and fold probably exist. (Section 

 0.) Still farther north, at &, section 4, a ridge of quartzite makes its ap- 

 pearance, indicating a decided increase of the fault. (Section 4.)t 



Again to the north the fault seems to be replaced by a simple fold, as 

 in section 3, h. To the north of Dry Creek the high schist ridge b, sec- 



* The details of this northern fold have been kindly furnished me by Mr. Holmes 

 and Dr. Hayden. 



t Though the structure at this point was at the time understood to be as given in 

 section 4, and though the quartzite is lithologically the same as neighboring archaean 

 quartzites, this ridge may still be a metamorphosed fragment of the somewhat similar 

 Cretaceous No. 1 broken down into the present position. 



