138 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



The anticline is succeeded on the west by the summit syncline which also 

 pitches north at a low angle. Like the anticline this fold shows numerous 

 local warps and, on the south, it is truncated by a strong east-west fault shown 

 on the map sheet. The western limb of the syncline shows a section through 

 nearly the entire Purcell series. Two miles east of the main fork of the Yahk 

 river the Creston beds have a sharp reversal of dips, indicating an anticline 

 broken by a longitudinal, north-south fault. The Yahk river is located in the 

 heart of this anticline. It may have been originally placed on the line of fault, 

 from which position the river has since slipped down tre dip an average 

 distance of two miles. To the west of the main fork of the river the dips 

 gradually change from an average of 45° W. to horizontality, and in the inter- 

 val, a great part of the Creston formation, the whole of the Kitchener and some 

 3,000 feet of the Moyie formation are exposed in succession. 



On the ridge overlooking the west fork of the river on the east, the dips 

 in the Moyie beds again become easterly, showing a narrow syncline which is 

 here only visible in this formation. Exactly on the line of the west fork the 

 Moyie strata are dropped down into contact with a gabbro sill which is intrusive 

 into the Kitchener formation. This west fork fault is remarkably straight in 

 the six miles through which, with unusual certainty, the outcrop of the fault 

 could be followed. The downthrow is, of course, on the east and may measure 

 more than 2,000 feet. 



From the west fork of the Yahk to Porthill nearly all suggestion of folding 

 is wanting and the relations are those of many fault-blocks. The dips are 

 highly variable, values from 5° to 80° or more being recorded. The dips are 

 generally much the higher in the narrower blocks. Here as in the Galton- 

 MacDonald system the fault-planes usually trend towards the north-northwest 

 and their dips seem invariably to approach verticality. 



The faults mapped between the west fork of the Yahk and the Moyie 

 river are among the most obscurely exposed of all. Others not shown on the 

 map sheet may be responsible for the duplication of the great gabbro sills in 

 this part of the Boundary belt. Much additional time and labour must be 

 expended before the full structure of this part of the belt will be declared. The 

 two blocks immediately east of the Moyie river are shown as separated by a 

 reversed fault along which the Creston quartzite has been driven up on the 

 back of the likewise steeply dipping and apparently underlying Kitchener 

 quartzite. A second interpretation is open, whereby the two formations are 

 regarded as in normal contact but both overturned to the west. 



The plane of the main fault at the Moyie river is nowhere exposed but the 

 relations of dip and strike are such as to leave no doubt as to the nature of the 

 displacement. The downthrow is to the west and is very great, probably 

 approaching 8,000, if not 10,000 feet. 



The fault running along the western base of the isolated mountain bearing 

 the Moyie sills is also believed to be mapped correctly. The downthrow is again 

 to the west but the displacement is probably no more than a couple of thousand 

 feet. 



