MIDDLE TO UPPER EOCENE SEDIMENTATION 569 



with heavy stream wash, consisting of subangular to rounded fragments 

 of granite and granodiorite, rocks which intrude the greenstone at a 

 distance of 1 to 2 miles back from the trench. This conglomerate repre- 

 sents alluvial fan or cone deposition from tributary valleys. The over- 

 lying arkose and shale, which are characteristically thinly bedded and 

 lack cross-bedding, are the products of deposition in quiet waters, with 

 very little or no current. The coal seams represent stages in the valley 

 filling when back sloughs and lakes were prominent, in which large 

 areas of the valley fill were at the level of the water and were covered 

 with a thick growth of evergreen and deciduous trees. Spasmodic recur- 

 rences of this condition and periodic downfaulting of the basin would 

 account for the accumulation of upwards of 2,000 feet of sediments, in 

 which are interstratified at least fourteen seams of coal. 



From the shales of the Chu Chua formation the writer collected several 

 suites of plant fossils, which were submitted to Prof. Edward W. Berry, 

 of Johns Hopkins University, for determination. Professor Berry places 

 them definitely in the Middle or Upper Eocene.* The Chu Chua forma- 

 tion, therefore, constitutes the first discovery of definitely established 

 Eocene sediments in the southern interior of British Columbia. Daw- 

 son,^ J)alj,^ Mackenzie,'^ Drysdale,^ and others report the occurrence of 

 similar groups of rocks, but they have been placed in the Oligocene or 

 loosely connected with some part of the early Tertiary record. 



Oligocene Diastrophism and Erosion 

 block-faulting 



Following a partial consolidation of .the Eocene sediments, the forma- 

 tion was block-faulted probably during the Oligocene, along lines run- 

 ning in northeasterly directions, parallel to the trend of the valley. As 

 a result of this faulting the rocks of the series are now found as strike 

 ridges on both sides of the trench, but dipping uniformly at 20 to 40 

 degrees toward the east. In other words, they dip away from the valley 

 side and into the trench on the west side of the river, and away from 

 the trench and into the valley side on the east side of the river. 



This diastrophism caused the rejuvenation of the river. Erosion by 

 the river then beveled the upturned edges of the sedimentary series. 



- Summary Kept., Geol. Surv. Canada, part A, 1921, pp. 85-8G. 

 5 Ann. Kept. Geol. Surv. Canada, 1894, pp. 66-76 B. 

 « Geol. Surv. Canada, Mem. 38, part 1, pp. 86-88. 

 ^ Geol. Surv. Canada, Mem. 87, pp. 31-39. 

 . 8 Summary Kept.. Geol. Surv. Canada. 1912, pp. 140-144. 



