ll'll I. C. RUSSELL — SURFACE GEOLOGY OF ALASKA. 



course of the river. The pebbles thua marked resemble glaciated pebbles so 

 closely that I took special pains to determine tbe origin of their peculiar 

 markings. Only the upper surfaces of the pebbles taken from the pavement 

 weir abraded. Moreover, uo pebbles showing the markings referred to were 

 found above high-water mark. That the pebbles were ground down, polished 

 and striated by the river ice passing over them during its descent of the river 

 is plainly apparent. 



Some of the Btones in this pavemenl arc angular masses of basalt, nearly 

 two feet in diameter. These, like the associated pebbles, are deeply abraded 

 and scratched in rudely parallel lines. On some of the rounded pebbles the 

 amount worn off on the abraded side was estimated to have been about half 

 an inch. 



Many of the Btones in this locality are so similar to glaciated pebbles that 

 it' removed from their normal position to a glaciated region, even the most 

 acute observer would attribute their markings to glacial action. When, 

 however, one knows the origin of the markings upon them it becomes evi- 

 dent that the scratches on the smooth faces are less regular and less firmly 

 drawn than the groove.- and striatums on typical glaciated pebbles. 



"Bowlder Clay" deposited by Rivers. — The Yukon, as already stated, 

 freezes deeply during the winter, and the ice near its borders, especially 

 where it is broad and .-hallow, rests on the bottom, and has large quantities 

 of stone and bowlders attached to it. All except the largest of the tributary 

 streams freeze to the bottom, and also furnish vast quantities of pebbles for 

 ice transportation. When the rivers break up in the spring, the ice with 

 it- loads of stone is floated down-stream, and, melting as it goes, distributes 

 pebbles and bowlders over the bottom of the river, and in places where at 

 other time- tine sedimenl i- deposited. In this manner it is conceivable that 

 a clay tilled with bowlders mighl he formed which would similate true bowl- 

 der clay in many ways. Certain bowlder clays along the Yukon and the 

 Lewes are described elsewhere in this paper, which, as there stated, may 

 have been formed in the manner here suggested. 



Old Deposits of ice-borne River Gravel. The pasl action of the river ice 

 in transporting stones is recorded by deposits of bowlders in lenticular masses 

 in the fine sedimenl exposed in the river bank-. Isolated bunches of gravel 

 wholly enclosed by fine sediment, and ten to fifteen feet below the surface, 

 are ool unusual in the caving river hank-. In -Mine places large bowldi 



i en in like -in mi ion-. These occurrences are satisfactorily accounted 

 for on the hypothesis thai the gravel and bowlders in question were trans- 

 ported and dep< -ihel by river ice. 



Flood-Plain I>>/><>ii-. The manner in which rivers build up, destroy, and 

 rebuild their Hood plain- can lie studied to advantage at many place- on the 



Yukon and Porcupine. The lower hundred mile- of the latter offers an 



