568 AV. L. UGLOW PEXEPLAIX IX BRITISH COLUMBIA 



valley of the ancestor of the present river as it flowed with a very low 

 gradient, bounded by relatively flat valley slopes, over the peneplain, 

 characterized by physiographic old age. The peneplain is early Eocene 

 or pre-Eocene (probably late Cretaceous) in age, and the depression is 

 the valley of the pre-Eocene (probably Cretaceous) N"orth Thompson 

 Kiver. 



Early Eocexe Uplift 



This peneplain was slowly uplifted in pre-Middle Eocene time. Since 

 Eocene sediments occur in the trench at a depth of 3,000 feet below the 

 upland, the uplift must have been approximately of that magnitude. It 

 was of sufficiently slow development to allow the river to maintain 

 throughout the course it had previously carved out for itself and to cut 

 its present trench beneath its former peneplain valley. The initiation 

 of the uplift can not be definitely dated, but it was sufficiently pre- 

 Middle Eocene to allow the river to cut down through 3,000 feet of hard 

 pre-Tertiary rock formations. A slow rejuvenation of the river accom- 

 panied the progressive uplift, and by the time of the commencement of 

 sedimentation, in Middle Eocene, the river had entrenched itself to a 

 depth of about 3,000 feet. The uplift was completed when sedimenta- 

 tion began, or by the middle of the Eocene. 



It is probable that this early Eocene uplift represented the Laramide 

 revolution as it affected the southern interior of the province. If so, the 

 Laramide revolution was one of slow progress which commenced in late 

 Cretaceous or very early Tertiary time and was completed before the 

 end of the Eocene. 



Middle to Upper Eocexe Sedimextatiox 



The Chu Chua formation preserves a fragmentary record of the topog- 

 raphy and geology of the trench bottom after it had been cut in Middle 

 to Upper Eocene times. As stated above, this formation consists of a 

 basal conglomerate varying in thickness up to 200 feet, evenly bedded, 

 highly feldspathic arkose, sandy shale, intraformational conglomerate, 

 and seams of coal. 



The lithology, stratigraphy, and structure of the upper part of the 

 formation indicate that it was deposited under conditions of still or very 

 sluggish water. The basal conglomerate is a tumultuous deposit char- 

 acterized by a heterogeneous mixture of large and small angular frag- 

 ments of the greenstone which is the adjacent country rock, interspersed 



