REPORT OF THE CHIEF ASTRONOMER 637 



SESSIONAL PAPER No. 25a 



It is certain that this differential erosion was long continued during the 

 Pleistocene period in each of the ranges where accordance of summit levels has 

 been discussed. Pleistocene glaeiation certainly tended to bring the high Front 

 ranges of the Rockies into accordance with the lower Galton and Purcell ranges. 

 There is every reason to suppose that like conditions and like results would 

 characterize still earlier glaciations. 



In summary, then, it may be said that partial explanation for summit-level 

 accordance is to be sought in a special, characteristic control of alpine climates. 

 In general, the climate of high levels is a glacial climate. In general, glacial 

 erosion is very great and the bulk of it is high-level erosion. In general, local 

 glaciers and glacial erosion are most abundant and long-lived about the highest 

 summits. One net result of glaeiation is to cause the specially rapid wastage of 

 those summits and to produce rough accordance among the peaks. 



4. The influence of the forest cap on summit altitudes. — Climate not only 

 breeds glaciers in the high levels of an alpine range; it normally determines a 

 more or less well-defined tree-line. The treeless zone is always more extensive in 

 area than the glacier-bearing zone, but the upper limit of trees is often not far 

 from coincident with the lower limit of the zone of cirque glaciers. It is logical 

 to find here a place for the theory that widespread accordance of summit levels 

 in an alpine range is related to the differential rate of erosion above and below 

 tree-line. The theory is so well known that it needs no special detailed state- 

 ment on the present occasion. Let it suffice to recall the principal reasons why 

 denudation is faster above tree-line than below, and once more note the inevitable 

 conclusion from that fact. 



a. Disintegration of rock. — Both as an evidence of incomparably more rapid 

 frost attack above tree-line than below, and as a condition for more effective 

 attack by agents other than frost, the ' Felsenmeer ' is significant. Illustrations 

 of this rock-chaos so characteristic of hundreds of peaks in the Boundary belt 

 are shown in Plates 32, 42 B, 70, and 71 B. 



b. Removal of rock-waste. — On the other hand, the streaming of weathered 

 material down the slopes is, other things being equal, probably several times 

 more rapid in the treeless zone than below it. 



(1) The direct beat and wash of the rain have practically negligible effect 

 on waste-removal below tree-line. The power of heavy rain washing the treeless 

 zone, either in the derived form of rills or as a sheet flood, is manifest to anyone 

 who has experienced a good shower above tree-line. 



(2) During the Boundary survey the writer has for the first time become 

 conscious of the importance of burrowing mammals in preparing loose rock-waste 

 for speedy transit to the valleys. In the western Cordillera field-mice, gophers, 

 moles, marmots, bears, and other species are each year doing an immense geologi- 

 cal work. There can be no exaggeration in saying that these burrowers annually 

 turn over hundreds of thousands, if not millions, of tons of soil or disintegrated 

 rock in either the Coast range or the Selkirk range of British Columbia. Such 

 work is of relatively little importance where mounds or fillings of snow tunnels 

 are protected by trees overhead. It is very different above tree-line, where even 



