REPORT OF TEE CHIEF ASTRONOMER 597 



SESSIONAL PAPER No. 25a 



pied the Fraser trough. On the disappearance of the ice the river has cut 

 away large tracts of the old delta, has built a clayey flood-plain over the erosion 

 surface, and is to-day pushing a new silty delta into the gulf. The old delta 

 is now represented by flat-topped remnants rising 200 feel or more above 

 sea. These extensive plateaus are bounded by sea-cliffs, and by the steep scarps 

 cut by the Eraser as its channels swing powerfully across its present flood-plain. 

 (Plate 61). 



Such are the conclusions to which the writer has come as a result of short 

 study of the gravel plateaus in 1901. Further field-work may, however, show 

 that their history has been, in some respects different; the problem is worthy 

 of special, more prolonged study. 



Summary. 



So far as they go, the observations made during the six seasons 

 of field work do not imply more than one period of glaciation. It does not 

 follow, of course, that there were not two or more distinct glaciations of the 

 boundary belt in Pleistocene time; the evidence on this point is as yet negative. 

 In a region of such strong topography we should hardly expect the deposits of 

 an earlier epoch to have been preserved if a later epoch of general glaciation had 

 intervened. The fresh condition of both rock ledges and drift deposits is so 

 similar to that observed in the eastern region of Wisconsin glaciation that one 

 is forced to the belief that the Cordillera was ice-capped in that latest phase 

 of- the Pleistocene. 



The Forty-ninth Parallel section is specially instructive as showing the 

 enormously greater erosive efficiency of local valley glaciers as compared with 

 the efficiency of a regional ice-cap. The ice-tongues of the Rocky Mountains 

 and of the Cascades have effected great changes in the forms of the mountains, 

 while much of the Cordilleran interior, though simultaneously covered by 

 ice of greater thickness, has suffered relatively little change in the pre- 

 Glacial topography. The difference of result is explained partly by the much 

 greater prevalence of bergschrund in the ranges affected by local glaciation; 

 and partly by the higher average velocity of the master local glaciers as com- 

 pared with that of the ice-cap. The comparison is specially illuminating since 

 both ice-cap area and local glacier areas were characterized by essentially 

 similar climatic conditions. In both cases snow-fall and rate of ablation were 

 much the same in these different mountain belts. The relative feebleness of 

 the Cordilleran ice-cap in erosive effect is fairly matched by the relative feeble- 

 ness of the Labrador ice-cap on the plateau-like surface of eastern North 

 America. In contrast to both stand the Pleistocene glaciers which lay in the 

 ChiHiwack and Chelan valleys, or those which occupied the Rocky Mountain and 

 Purcell trenches in late-Glacial time. All of these trough glaciers eroded the 

 living rock on a spectacular scale. 



The details given regarding the thickness of glaciers, direction of flow, 

 character of drift deposits, etc., are substantially similar in quality to those on 

 which Dawson based his generalization regarding the Pleistocene glaciation 

 25a— vol. iii— 39£ 



