REPORT OF THE CHIEF ASTRONOMER 279 



SESSIONAL PAPER No. 25a 



On both sides of the Pureell Trench, therefore, we have evidence of huge 

 displacements which have given this part of the Kootenay valley the character 

 of a fault-trough. The down-faulted block or blocks have, of course, lost much 

 substance through erosion but it seems most probable that the trench was 

 located in a constructional depression due to faulting. 



Another of the primary structural features of the Nelson range is the 

 unconformity at the base of the Irene conglomerate. The existence of the 

 unconformity is not conspicuously shown by contrasts of attitude between the 

 conglomerate and the older sediments. In fact, as above noted, the strike and 

 dip of the conglomerate and of zone A are often closely similar. The evidence 

 is more fully derived from (1) the much stronger metamorphism of the Priest 

 River terrane; (2) the abundant pebbles of Priest River rocks in the conglo- 

 merate; and (3) the truncation of zones A, B and C by the lower-contact plane 

 of the conglomerate. The Nelson range covers the only part of the Boundary 

 belt where the Rocky Mountain Geosynclinal is sounded to its full depth. 



Within the great Summit series monocline itself one of the most conspi- 

 cuous structural complications is the horizontal shift mapped as crossing the 

 divide between Monk creek and the south fork of the Salmon river. In the 

 field the effects of the shift are spectacularly clear. The almost vertical forma- 

 tions have been dislocated by a movement of about a mile along the vertical 

 west-northwest plane of shifting'. The relative displacement is that which 

 would have been produced if the southern block had moved westward through 

 that distance. The outcrop of the shift-plane could be readily followed for 

 four miles; its continuation westward across the southern slope of Lost moun- 

 tain is less evident in the field but seems competent to explain the relations of 

 the Pend D'Oreille limestones and schists to the quartzites on Lost mountain. 

 A second horizontal shift, not so evident, is mapped just south of Summit creek. 



About three miles west of the main divide of the range the upper beds 

 of the Summit series are duplicated for a great thickness by a powerful thrust. 

 This thrust is among the most remarkable elements in the anatomy of the 

 range. (Figures 17 and 18.) The plane of the thrust is stratigraphically located 

 in or very near the 225-foot band of conglomerate in the Dewdney formation. 

 The conglomerate has apparently acted, as a local plane of weakness. Along 

 that plane the entire overlying part of the Summit series has been driven 

 eastward and has then been pushed up on the back of the Lone Star schists. 

 Either during the thrusting, or, less probably, afterwards, the overridden and 

 overriding blocks together with the thrust-plane have been rotated so as now to 

 stand almost perfectly vertical or to show a slight overturning to the westward. 

 As a result the observer traversing the ridges on either side of Summit creek 

 will, on going westward, pass over the Dewdney beds, then the Ripple, Beehive, 

 and Lone Star in regular order, and will then, after crossing the thrust-plane, 

 pass over the upper Dewdney, the Ripple, Beehive, and Lone Star formations 

 once more. These relations are illustrated in Map 7 and in Figure 17. They are 

 specially clear on the high, nearly treeless ridges north and south of Summit 

 creek. The extreme northern and southern extensions of the thrust-plane are 

 not so well exposed and the mapping is there somewhat tentative. 



