RESEARCHES IN ALASKA 



541 



non-tidal Hidden Glacier could move 

 two miles under this impulse, however, 

 the Nunatak Glacier might be expected 

 to respond similarly, unless ( I ) this is 

 only a minor thrust from some tributary 

 or (2) the tidal condition introduces 

 some complication not previously ob- 

 served in studying glacial oscillations due 

 to this new cause. 



A GLACIER THREATENING A RAILWAY 

 BRIDGE 



On the lower Copper River is Childs 

 Glacier, which is seriously threatening to 

 destroy a steel railway bridge just com- 

 pleted. The rate of glacier motion in 

 Childs Glacier increased during the win- 

 ter of 1909- 19 10 so that part of the mar- 

 gin of the glacier changed its forward 

 movement from nothing to two and as 

 much as eight feet a day. 



This glacier, as the map shows (page 

 543), is the smallest of three great ice 

 tongues — Miles, Childs, and Allen gla- 

 ciers — projecting into the Copper River 

 valley. Around these three the Copper 

 River winds through a succession of 

 rapids and lake-like stretches. The Cop- 

 per River and Northwestern Railway has 

 just been built between Childs and Miles 

 glaciers and on over 5^2 miles of the 

 stagnant ice of Allen .Glacier. Childs 

 Glacier is 10 to 12 miles long, not much 

 over a mile wide in the mountain valley, 

 but it widens to over three miles in Cop- 

 per River valley. 



Its front is a precipitous white wall 

 250 to 300 feet high, and is swept at the 

 base by Copper River. How a small 

 section of the front of Childs Glacier 

 compares with the Capitol building at 

 Washington is shown in the photograph 

 on page 546. The map on page 543 

 compares Childs Glacier graphically with 

 the Nisqually Glacier, one of the largest 

 ice tongues on Mount Rainier, and Miles 

 Glacier with four of the largest and best- 

 known glaciers of the Canadian Selkirks 

 and the Glacier National Park in the 

 Montana Rockies. 



In August, 1909, Childs Glacier was 

 advancing at about its normal rate — four 

 feet a day at a point near the north side 

 and perhaps six or seven feet a day in 



midglacier. The melting and the many 

 icebergs discharged from the terminal 

 cliff at that time just about balanced this 

 advance, so that the front of the glacier 

 remained in about the same place. Near 

 the north margin there was no advance 

 when we saw Childs Glacier in 1909, 

 and probably there had been none since 

 1905 and perhaps none to speak of since 

 1884, though its relationship to the river 

 had varied slightly during this period, 

 and slight crevassing had begun in 1906 

 and 1909 where there was none in 1905. 



During the winter and early spring of 

 1909- 1910, however, the glacier began to 

 advance more rapidly, buckling up the 

 ice of the frozen river. In June, 1910, 

 the ice front had moved forward from 

 920 to 1,225 feet, narrowing the river 

 to 400 or 500 feet. 



Every time the ice cliff was sufficiently 

 undercut by the river, great masses of 

 ice would cascade down the front, rais- 

 ing a gigantic wave in the river. People 

 in Alaska speak of the discharge from 

 the front of Childs Glacier as "slough- 

 ing." A "slough" has always raised 

 waves in Copper River, making it dan- 

 gerous to shoot the rapids in front of 

 Childs Glacier in a boat, or to line a boat 

 up the opposite bank ; but in the spring 

 of 1910 the conditions were accentuated 

 by the advance of the glacier and the 

 pushing of the river eastward. 



During the advance the waves washed 

 up over a bank 5 to 25 feet in height and 

 rushed back 100 or 200 feet into the 

 alder thicket. Ice blocks up to 10 tons 

 in weight were thrown in among the 

 trees. Stones a foot or two in diameter 

 were hurled into the thicket. Alders 9 

 to 1 1 inches in diameter were stripped 

 of leaves and bark and bent backward 

 or broken off short, or uprooted or 

 buried beneath the gravel and boulders 

 and macerated trunks of other trees. 



The river bank, which was cut back 

 some in the preceding year, was in 1910 

 being fairly eaten up by the iceberg 

 waves which crossed the river, 50 to 60 

 feet by actual measurement having been 

 removed along the bank of the stream 

 facing the glacier. 



