236 



Ashcroft would imply continental conditions and con- 

 sequent erosion during at least late Cretaceous time. 



3. South of Kamloops lake at an elevation of about 

 2,000 feet (610 m.) ar extensive flat is underlain by 

 Jura-Triassic rocks, and entrenched by an early 

 Tertiary river valley. The old river valley is filled with 

 Coldwater conglomerate, sandstone, and shale dipping 

 at low angles. The rocks of this formation form prominent 

 strike ridges which rise high above the flat referred to, 

 and form, at the contact, a topographic unconformity. 

 The flat is a conspicuous topographic feature, and is 

 thought to represent a remnant of an old uplifted Cre- 

 taceous erosion surface since modified by Glacial action. 

 Here, through favourable tectonic conditions, a portion 

 of the Cretaceous erosion surface has been preserved to 

 the present time and dominates the topography. 



4. The next erosion cycle provisionally referred to pre- 

 Miocene and post-Eocene time, is evidenced by a marked 

 unconformity between early Tertiary formations and lower 

 Miocene (?) volcanics. Near Ashcroft, as elsewhere through- 

 out the Belt of Interior Plateaus, the early Tertiary form- 

 ations are strongly uptilted, and they have been apparently 

 subjected to crustal disturbances prior to the later vulcan- 

 ism. Such orogenic movement would naturally inaugurate 

 a new cycle of erosion which probably removed vast 

 quantities of the loose continental deposits of early 

 Tertiary age. 



5. The third and most important erosion cycle which 

 is thought to have largely developed the present upland 

 topography, continued into the Pliocene. 



The Miocene (or Oligocene?) lavas which cap the hills in 

 so many widely scattered localities throughout the Belt 

 of Interior Plateaus, have been warped to form broad 

 synclinal basins and anticlinal domes. The anticlinal domes 

 have since been removed through denuding agencies. It 

 is found that the present late mature upland (locally a 

 peneplain) truncates or bevels the tilted lavas for great 

 distances. The upland erosion surface in this district may 

 be correlated with one found by the writer during the 

 summer of 191 1 in the Franklin Mining district in the 

 Columbia Mountain system. There it truncates the 

 Midway Volcanic group of trachytes and alkalic basalts 

 referred to the Miocene period. 



