GLACIATION 139 



Pleistocene gravels. These features, while not demon- 

 strative without further study, clearly suggest that the 

 Columbia may formerly have followed the structural val- 

 ley northward to and through Puget Sound, reaching the 

 ocean by way of Fuca Strait. The occupation of the 

 strait and sound by the great Pleistocene glacier would 

 have compelled the river to find some different course, 

 and when it had once carved a channel through the coastal 

 hills the filling of its previous channel by glacial gravels 

 would prevent its return to the earlier course. 



In view of the possibility that the lower course of the 

 Columbia dates only from the Pleistocene, it is evident 

 that the character of its estuary has no decisive bearing on 

 the problem of pre-Pleistocene base-level. 



Summary. Before the great glaciers of the Pleistocene 

 began their work the district included a varied topography. 

 The larger part was mountainous in the ordinary sense, 

 with crests at various heights and a complicated system of 

 steep-sided ridges, spurs and gorges. There were exten- 

 sive remnants of a high-lifted peneplain, its plateaus mark- 

 ing the areas of most resistant rock, and above these 

 plateaus rose summits of the nature of monadnocks. 

 There were remnants of a low peneplain a peneplain 

 which is now near sea-level and these occupied areas 

 of relatively weak rock. There was a system of river 

 valleys or master lines of drainage, narrow where the 

 rocks were most resistant and more open among weak 

 rocks. The bottoms of these valleys were in part below 

 tide-level. 



Glaciation 



Rounding of Angles. Turning now to the results of 

 Pleistocene ice erosion, one of the most evident is the 

 rounding of salient features. Where the slopes of moun- 

 tain spurs have an average inclination as steep or steeper 

 than that which permits the resting of talus, the inter- 



