237 



GLACIATION. 



The deep pre-Glacial Thompson valley contains a great 

 thickness of fluvio-glacial material now in process of being 

 excavated. Sections of such glacial and interglacial debris 

 exposed by the river and railroad, aid considerably in the 

 determination of the Pleistocene history of the province. 



The region of Interior Plateaus was, during the Pleisto- 

 cene, covered by the Cordilleran ice cap, whose direction 

 of flow here, as shown by striae, was about S. 35 E. 

 The upland slopes are thickly mantled with morainic 

 drift and erratics left stranded by the retreating ice sheet. 

 On the other hand, the contemporaneous boulder clay 

 deposited in the valleys below, has since been largely re- 

 moved by the advance of valley glaciers. 



With its waning, the Cordilleran continental glacier 

 gave place to alpine, cirque, and valley glaciers. Much 

 englacial and superglacial material was deposited and 

 reworked by water. The older gravels, sands, and strati- 

 fied clay silts, capped by boulder clay, are referable to 

 this period of alluviation, contemporaneous with the first 

 period of valley glaciation. 



The valley ice slowly retreated until the time of the 

 maximum extension of the Keewatin ice sheet on the 

 east, when the second period of valley glaciation in the 

 Cordilleran belt probably took place. This advance 

 of the ice removed much of the older morainic and outwash 

 materials, deeply eroded the valleys, and heaped up 

 lateral and terminal moraines. The high-level esker- 

 like ridges of the valley sides probably represent the work 

 of streams at the borders of the ice. The streams draining 

 the ice front carried down and deposited large quantities 

 of land waste in the form of a deep alluvial fill. 



With the melting and recession following the maximum 

 advance of the second period of valley glaciation, large 

 amounts of drift materials were set free. Great thick- 

 nesses of silts were then deposited in the tranquil waters 

 of lakes. These lakes were formed on the main valley floors, 

 either dammed by powerful local glaciers entering from the 

 sides, or perhaps locally basined at a time of special sub- 

 sidence yet greater than that recorded for the late Pleisto- 

 cene along the Pacific shore. At the mouths of tributary 

 creeks alluvial fans composed of cross-bedded gravels 



