STRIATIONS IN GRAVEL BARS 



77 



Fig. 2, The force of the water would tend to reheve the roots of any 

 material competent to sink the tree, such as rocks or frozen earth. 

 The only other material at hand to which one might ascribe such a 

 force, is ice. 



There is evidence that during the ice breakup of spring, great 

 pressure is exerted on the banks and shallow portions of the stream.^ 



At a number of places last summer the writer observed talus where 



Fig. I 



rock fragments were pressed into a pavement. These were seen both 

 along the Yukon and along the Porcupine rivers. RusselP in his 

 paper on the surface geology of Alaska speaks of the river ice as 

 producing pavements of pebbles along the banks and of the pebbles 

 being faced and striated on their upper side. These pavements occur 

 between the high-water and low-water hnes, and as the water was low 



1 For an account of the "breakup" see F. C. Schrader, "Professional Paper," 

 U. S. Geological Survey No. 20, 1904, pp. 15, 16. Also see Stoney, Naval Explora- 

 tion in Alaska, U. S. Naval Institution, Annapolis, Md., 1900, p. 52. 



2 Bulletin oj the Geological Society of America, Vol. I, 1890, pp. 119, 120. 



