564 ^y. l. ugloav — peneplaix in British Columbia 



and rocky ridges stand as monadnocks in high rehef above its general 

 level. 



In places where the valley consists of a single depression it is broad 

 and up to 3 miles in width at its base. In other places, as near Louis 

 Creek and Mount Olie, the valley consists of two or more longitudinal 

 trenches, separated from one another by high ridges, whose tops repre- 

 sent isolated remnants of the plateau. In these cases the river is found 

 to flow through one of these subsidiary trenches and is consequently 

 quite constricted (see figure 2). 



General Geology of the Trei^ch ^ 

 the rock formations 



The rock formations of the vicinity may be divided into three main 

 groups: (a) the pre-Tertiary complex, {h) the Tertiary sedimentary 

 and volcanic series, and (c) the post-Tertiary lava flows and unconsoli- 

 dated sediments. 



PRE-TERTIARY COMPLEX 



This consists of several series of metamorphosed rocks, chiefly sericite, 

 chlorite and hornblende schists, slates, quartzites, crystalline limestones 

 with highly altered volcanic tuffs, and pillow lavas, cut by many stocks, 

 sills, and dikes of gabbro, granite, grandiorite, and a batholith of granite. 

 No fossils have yet been found in the rocks of the complex, so that it is 

 not known whether they belong individually or collectively to the Pre- 

 cambrian or the Paleozoic. The age of the intrusive rocks has not been 

 definitely determined. Some of them may be of Precambrian age, some 

 of them are believed to be Jurassic, and others may even be Tertiary. 



The attitudes of the foliated or bedded rocks are not parallel to the 

 main northwesterly Cordilleran trend. They strike east, northeast, or 

 northwest with very little tendency to exhibit any regularity. 



TERTIARY ROCKS 



These are of two ages — Middle or Upper Eocene and Miocene. 



The Eocene, or Chu Chua formation, consists of a series of basal con- 

 glomerate (or agglomerate), highly feldspathic arkose, sandy shale, and 

 sub-bituminous coal. These rocks occur in the bottom and along the 

 lower slopes of the Xorth Thompson trench, in isolated erosion remnants, 

 the highest exposures found being at an elevation of about TOO feet above 

 the level of the river (see figure 4). They underlie a few narrow low 



For full discussion see Summary Rept. Geol. Siirv. Canada, part A, 1921, pp. 72-106. 



