284 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



J. Erosive Pozver of Ice. 



Prof. Albert Heim,^ a thorough student of glacial geolog}^ 

 writes: — 



Advancing glaciers often leave loose detritus undisturbed [where the gla- 

 cial valley widens]. Under other circumstances, particularly in contracted 

 valleys where there are obstructions, the advancing glacier plows up the 

 ground down to solid rock. * * * It is only loose masses standing very 

 much in the way which were pushed along by the glacier. 



Whitney^ and Irving^ likewise ascribed very little erosive 

 power to glacial ice. 



Geikie,^ Bell,^ and others apparently are of the opinion 

 that glaciers considerably modify the shape of the masses 

 of rock over which they pass. Indeed, that they do so is 

 obvious to anyone who observes the rounded forms of the 

 roches moutonnees, characteristic of the beds of glaciers. 

 The scouring and pohshing work of the ice is admitted by 

 all observers, but all this is a very different matter from 

 the gouging out of canyons by ice, as supposed by Muir 

 and others. Indeed Heim and Becker regard the ice sheet 

 as causing the relative suspension of valley formation. 

 Becker considers that the former ice-cap of the higher part 

 of the Sierra Nevada in a large measure protected from 

 erosion the underlying rocks. On the other hand, it is clear 

 that the silt-laden waters formed by the melting of a glacier, 

 chiefly near its terminus, would form an active erosive agent, 

 and this stream would not begin at the edge of the ice-mass, 

 but further up the canyon under the glacier. 



As stated by Geikie,'' an approximate measure of erosion 

 by glaciation may be obtained from an estimate of the 

 amount of sediment in the glacial streams and from the size 

 of the moraines. The average quantity of sediment dis- 

 charged from the melting end of a glacier during a year 

 having been estimated, it would be easy to determine its 



1 Handbuch der Gletscherkunde, 1885. The quotation is taken from Becker's paper on 

 " The Structure of the Sierra Nevada," p. 65. 



2 Climatic Changes, 1882, p. 8. 



3 Quart. Journ. Geol. Soc. London, 1S83, pp. 62-71. 

 < Text-Book of Geology, 1893, pp. 429-431. 



6 Bull. Geol. Soc. Amer., Vol. I, pp. 287-310. 

 ''Text-Book of Geology, 1893, p. 432. 



