638 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



the weak veneer of turf is broken in the burrowing, and where the millions of 

 mounds or tunnel-casts are exposed to every agent of transportation. 



(3) The transporting efficiency of wind in the treeless zone of lofty moun- 

 tains has, on the whole, been more emphasized by European observers than by 

 those of America. So far as this is the case, Europeans have come nearer to 

 the highland view than we have in this country. The summer quiet of alpine 

 summits of itself gives a most deceptive idea of the power of wind in the 

 heights. During the other seasons winds of almost hurricane violence are far 

 from uncommon, if we can generalize from the limited instrumental data so far 

 issued from high-lying observatories. We may believe that dust, sand, and fine 

 gravels are so rare above tree-line largely because of such winds. For obvious 

 reasons, sand-blasting there plays no such role as it does in the sculpturing of 

 rock-forms in lowland deserts ; but transportation by the wind is another influence 

 placing in strong contrast the conditions of erosion in the regions above and 

 below tree-line. 



(4) Erosion and transport through avalanches are enacted in both the tree- 

 less and the forested zone. In the lower zone the destruction wrought by a great 

 avalanche may be great, but it is largely a ruin of tree-trunks. In the lower 

 zone the avalanche paths are tolerably well fixed from year to year, sparing much 

 the greatest part of the forested area. In the treeless zone, avalanches have 

 generally less momentum, but they are more numerous, less localized, and there- 

 fore more likely to find and sweep down loose rock debris. Above tree-line their 

 ruin is wholly rock-ruin. It seems safe to conclude that snow-slides are more 

 powerful agents of degradation above tree-line than below. 



(5) The general streaming and cascading of rock-waste under the direct 

 pull of gravity are evidently immensely more rapid in the treeless zone than 

 where the strong vegetation mat binds humus, soil and boulder to the bed-rock, 

 though it be without perfect, ultimate success. The fine-grained felsenmeers 

 of the more friable peaks in the Boundary belt were often found to be mobile 

 under very slight pressure, such as that of a man's foot. Many of the summits 

 are ornamented with terracelets of loose debris which tends to stream down the 

 slope but is held in front by bands of turf. Many of these small tongues of 

 rock-waste are moving on slopes as low as one or two degrees. Similar forms 

 in great abundance may be seen in the treeless parts of Labrador and Alaska, 

 and again often on relatively flat slopes. Hundreds of the scallops, each covering 

 a few to many square yards may be counted on a single summit in the Front 

 ranges or in the Okanagan range — in all cases above tree-line. After six seasons 

 of work on the Boundary belt the writer feels convinced that the mere stream- 

 ing of waste in the treeless levels of the Cordillera is competent to produce toler- 

 ably wide and flat crests on the mountain ridges, though it is not clear that the 

 wind is not even more competent. Perhaps the finest illustrations of the com- 

 bined effects of these two agencies are to be seen on the granite summits of the 

 Okanagan range, where miniature plateaus have often been developed above 

 tree-line. The writer suspects that the influence of the tree-line has, in fact, 

 been responsible for some of the ' peneplain remnants ' mapped by Willis and 

 Smith. 



