GLACIATION 
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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- 
