203 



valley of first magnitude. Probably during 

 the late Pliocene the region was uplifted and 

 a narrower trough sunk in the old valley floor. 

 Remnants of that floor are visible in the trench 

 at elevations of 650 to 1,000 feet (200 to 300 m.) 

 or more above the river. The bed-rock form 

 has been seriously affected by Glacial erosion 

 and deeply covered with drift, into which the 

 Columbia has cut, with the development of 

 terraces and a broad flood-plain. 



The Pleistocene deposits are so thick and 

 continuous in the trench that bed-rock crops 

 out at the railway only twice between Golden 

 and the 53rd mile-post, a distance of 28 kilome- 

 tres. Practically as far as the observer at Golden 

 can see on the southwest side of the trench, 

 both north and south of the town, the rocks 

 are silicious sediments of latest Beltian age. 

 On the northeast side, the heights are chiefly 

 composed of the Silurian (Moberly Peak) or 

 Ordovician formations. After leaving Golden 

 the first important exposure of rock at the 

 railway track is on the right, at the crossing 

 of Blaeberry river (45 mis.), where the Goodsir 

 (Ordovician) shales are dipping at an angle 

 of about 55 to the northeast. These, like all 

 the other Paleozoic strata seen in the trench, 

 are more or less crumpled and cleaved, indicating 

 great disorder in this broad band followed by 

 the trench. On the whole, however, the 

 Cordilleran strike is preserved here, as it is all 

 across the Middle ranges as far as Albert 

 Canyon. 



Purcell Mountain System. — The rugged 

 wall on the west side of the trench is the north- 

 easterly limit of the group of the high peaks 

 here included in the Purcell system. As shown 

 in the accompanying structure section, the 

 rocks in this escarpment form the northeastern 

 limb of a wrinkled syncline adjoining a well 

 developed anticline along which the valley of 

 Quartz creek has been excavated. The com- 

 pound syncline has suffered intense glacial 

 erosion, producing abundant alpine horns; 



