11^ 1. t . RUSSELL URFACE GEOLOGY OF A.LASKA. 



Bowlders Transported by River lee. — The first large bowlder that I .saw in 

 ascending the Yukon, the travels of which couhl be approximately measured, 

 was mi thr let! bank of the river, about fifteen miles above Nbwikakat. This 

 is a granite bowlder, measuring 4 by 3 by •">■] feet. The ledge from which it 

 ii i u~t have been derived is in the Lower Ramparts, about one hundred miles 

 above its present position. < >ther bowlders, many of them larger than this, 

 were seen al many localities, but the distances they had traveled were not 

 ascertained. The largest one measured was near McGrath's Station. It is 

 composed of dark, volcanic rock, is rudely spherical, and measures a little 

 over six feet in diameter. 



Bowlders were frequently observed just above high-water mark, where the 

 river banks are low and composed of sand and gravel. These had evidently 

 been forced landward by ice pressure during the breaking up of the river in 

 spring. The furrows plowed during their advance, as well as the hank of 

 sand and gravel accumulated in front, could still be distinguished. The 

 force which moved these bowlders was plainly the river ice. When the direc- 

 tion of movement could be determined, it was always found to have been 

 down Btream, but at the same time trending away from the river at an angle 

 of from 30 to perhaps 50°. The direction of movement, as well as the fact 

 that the bowlders occur at high-water mark, and often a little above that 

 horizon, shows that they must have been disturbed at the time when the river 

 was at it- flood stage, and expanding so as to force ice over its hanks. 



It is well known that when a river is rising the drift-wood it carries ten Is 

 to travel towards the shores, and frequently become.- entangled in the vege- 

 tation on the hanks. When falling, the drift -wood tends towards the line of 

 swiftest current. A similar rule controls the direction taken hy the floating 

 ice during spring freshets. I have been informed by persons who have wit- 

 nessed the breaking up of the Yukon in spring, that ice in immense cakes is 

 frequently forced up on the shore to a height of ten or fifteen feet, and re- 

 main- long after the river ha- fallen and is clear of ice. It is during the 



:umulation of such ice heaps that bowlders are moved in the manner de- 



ibed abov.e. Scars and marks of abrasion, due to ice, are frequently seen 

 ■ hi tne trunks al a height of ten feel or more above tic high water line of 

 the river. 



Furrows in the sands of the river banks which had been formed by blocks 

 ice forced shoreward in the same manner as the bowlders just described 



were observed at many localities. In these instances the shape.- of the ice 

 cake.- could he clearly distinguished in the banks of -ami. frequently three 

 ■ or more in height, that had been forced up in front of them. Prom the 

 manner of formation it is obvious that the furrows made by bowlders and 

 ice as just described are transient features, obliterated ami renewed at each 

 iking up of tin- ri\ 



