REPORT OF TEE CHIEF ASTRONOMER 581 



SESSIONAL PAPER No. 25a 



as well as the head of a mountain glacier, the valley walls are gradually 

 steepened, giving cross-profiles of the same quality as those in the yet more 

 spectacular amphitheatres up-stream. In other words, trough and fiord erosion 

 as well as cirque development are notably conditioned by the processes operating 

 in the bergschrund. 



But it may be quite wrong to attribute the greater part of the erosion in the 

 average trough to plucking. The field studies of Eeid, Hess, and others have 

 shown that glacial grinding, with the formation of rock-flour, is competent to 

 deepen a glacier-filled valley with relatively great rapidity. Eeid concluded 

 that the average amount of fine sediment contained in the streams draining 

 Muir glacier corresponds to an annual loss of about three-fourths of an inch of 

 rock over the whole bed of that ice-sheet.* Analogous results have been obtained 

 from various studies of the small sheets in the European Alps.f Russell has 

 shown that the coarse morainal deposits formed by the Pleistocene valley 

 glaciers of the Sierra Nevada of California are truly insignificant when com- 

 pared with the amount of rock which must have been removed to shape the 

 many U-shaped troughs mouthing in Mono valley.:}: He concludes that the 

 share of glaciers in this sculpturing work was small, but indicates the possibility 

 that ' other observers, it is true, might give a much higher estimate for the 

 amount of fine material deposited in distant parts of the lake, and conclude 

 that profound glaciation had occurred.' Such observations as those of Reid and 

 Hess suggest that this second view is the more probable one. The recently 

 published, superb map of the Uinta mountains, together with Attwood's 

 accompanying Glacial monograph, shows the reality of Pleistocene glacial 

 erosion on a great scale in that range. § The results of bergschrund erosion are 

 there strikingly similar to those observed in the Clarke and Lewis ranges (see 

 especially plates 2, 7, and 8 of the monograph). On the other hand, the moraines 

 at the piedmont slopes of the Uintas are much too small to match the volume 

 of rock which has clearly been removed from their troughs by glacial erosion. 

 The missing material must be sought in the fine silts occurring in broad sheets 

 far outside the range. 



In view of these and many other observations made by glacial experts the 

 writer has come to the conclusion that glacial scour or abrasion proper has been 

 largely, if not chiefly, responsible for the demonstrably great glacial erosion in 

 the low-gradient portions of the master Cordilleran valleys. 



The conclusion in no wise conflicts with the obvious fact that the great ice- 

 caps such as the Pleistocene Labrador sheet, performed comparatively little 

 erosion of any kind. The controversy as to the efficiency of glacial erosion has 

 been prolonged partly because insufficient emphasis has been placed on one 

 thorough contrast between ice-caps and mountain glaciers. The former has 

 outflow on all or nearly all radii; the latter have outflow in restricted channels. 

 The feeble excavating power of the larger and generally much thicker sheet is 



* H. F. Reid, National Geographic Magazine, Vol. 4, 1892, page 51. 

 t H. Hess, Die Gletscher, 1904, page 179. 



X I. C. Russell, Eighth Annual Report, U.S. Geol. Survev. pt. 1, 1887. p. 349. 

 § W. W. Attwood, Prof. Paper No. 61, U.S. Geol. Survey, 1909. 

 25a— vol. iii— 38J ' 



