LESSONS OP LITTLE YOSEMITE VALLEY 731 



these structures and traussects both kinds of rock. Were abrasion the chief 

 process, the ice would have worlced with practically equal facility everywhere, 

 regardless of structural differences, but the very oi)posite has manifestly been 

 the case. It has achieved large results where the rocks were mostly fissile, 

 but in massive rocks not susceptible to plucking and reducible only by abra- 

 sion, it has accomplished little indeed. Both in its major and in its minor 

 lineaments, the valley exhibits striking evidences of this selective action. 

 Thus, while it has been broadly opened for some distance, it is blocked at its 

 lower end by massive domes that have remained standing like gigantic roches 

 montonnecs, although overtopped by the ice of at least two glacial periods. 

 At its upper end, again, it contracts as the massive granite becomes more 

 prevalent, and finally is reduced to a narrow winding gorge. These and other 

 features are valuable as indices by which the efficiency of the ice as an eroding 

 agent may be gauged. 



TWO GLACIERS IN ALASKA 



BY LAWRENCE MARTIN 



(Al)stract) 



Among the glaciers studied by the National Geographic Society's Alaskan 

 expedition of 1910 are two of unusual activity. Columbia glacier, in Prince 

 William Sound, Alaska, began to advance in 1908, and has been visited on 

 July 15, 1908, June 24 and August 23, 1909, June 30 and September 5, 1910, 

 during which time its front has been progressing at an undetermined rate 

 where tidal, but on an island and at the borders at the rate of from .9 to 2.1 

 feet a day, destroying forests and peat bogs and modifying marginal drainage 

 and marine deposits. Childs glacier, on Copper River, was advancing at about 

 its normal rate in August, 1909, the ice melting or discharging as icebergs into 

 Copper River sufficiently fast so that the front remained nearly stationary. 

 During the winter of 1909-1910, the rate of motion increased, a previously stag- 

 nant, shrub-covered part of the margin visibly advancing into the forest at the 

 rate of 2 to 8 feet a day during June to October, 1910, when it was re-mapped 

 at frequent intervals. Independently of this increased rate of advance, the 

 position of the ice front in the river has oscillated during the summer with 

 the stages of water in Copper River. A $1,400,000 steel bridge and the key to 

 a railway system is threatened by this advance. 



Discussion 



Prof. R. S. Tarr asked whether the ice of the advancing glacier pushed 

 beneath the soil and forest or whether it merely shoved the soil ahead. 



Professor Martin replied as follows: No such projections of the advancing 

 ice front were seen, but their presence was suggested by the overturning of 

 trees before the glacier reached them and by the carrying forward of material 

 as if on the projecting snout of a glacier. 



Professor Martin replied as follows to a question by Mr. Brooks : There is 

 present movement in the Allen (formerly Baird) Glacier, faster in the clear 

 ice portion near the mountain valley than in the expanded bulk, covered with 

 moraine and vegetation and traversed for 5 miles by the railway. 



