OLIGOCENE DIASTROPHISM AXD EROSION 571 



the difference of elevation of the upland on the east and west sides of tho 

 trench. 



Erosion, accompanying and following the diastrophism, succeeded in 

 removing a large part of the lava cap and underlying Eocene sediments 

 from the valley. The only portions of the Miocene lavas which re- 

 mained at the end of the Pliocene consisted of rock terraces rising to 

 heights of 800 feet above the bottom of the valley. The central portion 

 of the trench was reexcavated to a depth about equal to that which ob- 

 tained at the inception of Eocene sedimentation. 



PLEISTOCENE Q LACTATION AND SUBSIDENCE 



A Pleistocene ice-cap which filled the valley and extended to a height 

 sufficient to glaciate and groove rock hills at an altitude of 7,500 feet 

 covered the southern interior and flowed in a general southeasterly direc- 

 tion. The ice removed the weathered rock from the ridges, deposited 

 erratics and till, and produced a "roche moutonnee" effect on the rocky 

 slopes of the valley and intratrench ridges (Mount Olie, Queen Bess 

 Eidge, Fennell Mountains). It supplied to the glacial river great quan- 

 tities of boulders, sand, silt, and mud, which were deposited lower down 

 as a thick valley fill. Thick deposits of wdiite silt (the "White Silts" of 

 Dawson*^) were deposited at this time, covering the glacio-fluvial gravels. 

 Remnants of these may now be seen as terraces up to altitudes of 2,250 

 feet along the sides of the North Thompson Valley. The presence of 

 these silts on top of the glacio-fluvial gravels indicates a submergence of 

 the land surface, probabl}^ below sealevel. 



RECENT UPLIFT, EROSION, AND VULCANI8M 



Following the period of the deposition of the "White Silts'" there was 

 a considerable uplift of the southern interior, which caused a rejuvena- 

 tion of the streams. The North Thompson Valley was reexcavated in 

 the unconsolidated Pleistocene sediments, and the "AVhite Silts" and 

 glacio-fluvial deposits were carved into a series of finely developed hang- 

 ing terraces. The eroded portions of the unconsolidated formations were 

 reasserted in Recent times and are now found constituting the lower 

 benches and river flats, which are such marked features of the valley 

 bottom. As a result of this post-Pleistocene or Recent erosion, the North 

 Thompson Valley was again deepened so that its 1)ottom is now in a great 

 many places at approximately the same stratigi-aphical deptli as it was 

 in Middle Eocene and Pliocene times. 



"Ann. Kept., Geol. Surv. Canada, vol. vii. 1S04. pi). liH.'M".)! I'>. 



