STRIATIONS IN GRAVEL BARS OF THE YUKON 

 AND PORCUPINE RIVERS, ALASKA^ 



V. H. BARNETT 



The presence of furrows in gravel bars of spreading and meander- 

 ing streams in Alaska seems not to have been mentioned in the litera- 

 ture on Alaskan geology. 



These channels may be seen on the extensive bars of the Yukon 

 and of the Porcupine rivers throughout the Yukon Flats. The 

 bars are remarkably well developed along the Porcupine for about a 

 hundred miles above its union with the Yukon and in low water they 

 are exposed as broad, gravelly, and sandy beaches from one to five 

 miles in extent. These extensive bars give excellent opportunity to 

 observe the striations. An uprooted tree is often seen lodged on a 

 bar with a channel marking its trail (Fig. i). 



Three hypotheses may be offered in explanation of these furrows : 

 first, that they are caused by tree trunks held firmly to the river bottom, 

 either by accumulated debris or ice, and moved forward by the force 

 of the stream; second, that they are caused by blocks of ice beneath 

 a load or ice jam and moved forward in the breakup; or, third, that 

 they are due to trees passing over bars only partly supported by 

 water, the current banking up behind the stump, though having suf- 

 ficieat force to push slowly along, yet not able to remove the marks of 

 the tree. 



The third method would seem a simple explanation for the origin 

 of the furrow shown in Fig. i. Here an uprooted pine tree may 

 be seen at the down-stream side with a straight furrow passing up 

 stream. 



It does not seem to the writer that any accumulation of debris 

 that might collect in the roots of a tree would be competent to hold 

 it to the bottom with sufficient force to make the furrows shown in 



I Published by permission of the Director of the U. S. Geological Survey. 



76 



