BOWLDEE (LAY OF DOUBTFUL ORIGIN. 143 



The freshness of glaciated surfaces in the North West Territory, in southern 

 Alaska, and in the High Sierra of California merits attention. It may be 

 suggested in this connection that the glaciers on the west coast of North 

 America were not contemporaneous with the Pleistocene glaciation of the 

 northeastern part of this continent, but of much later date. 



Bowlder Clay. — In the valley of the Yukon, between Rink rapids and 

 Lake Lebarge, there is a deposit of bowlder clay some twenty-five to thirty 

 feet thick, exposed in the scarps of the terraces bordering the river. That 

 this is a true bowlder clay deposited by glaciers is accepted without question 

 by both Dawson and McConnell. It is a light-brown earthy deposit, quite 

 homogeneous in composition, but sometimes obscurely stratified, and con- 

 tains pebbles and bowlders, some of them striated, scattered abundantly 

 through it. It occurs just below a region that bears undisputable evidence 

 of ice occupation, and has unquestionably the characteristics of a true gla- 

 cial deposit. To doubt that it was deposited directly by glaciers may seem 

 hypercritical ; but there are good reasons for believing that a very similar 

 deposit is now forming in the Yukon and other northern rivers, owing to 

 the transportation and deposition of gravel and bowlders by river ice. 



The bowlder clay along the Yukon is apparently confined to the river 

 valley and does not cover the adjacent hills. At least I could not satisfy 

 myself that it extends back from the river, as would be expected had it 

 been deposited by a broad ice sheet. The bowlder clay along the Yukon 

 occurs only below Lake Lebarge. Above that lake the lacustral deposits, 

 which are a continuation of these resting on the bowlder clay lower down 

 stream, have been dissected by the river to a depth of 150 feet or more, and 

 in some places, as at Miles canon, to the underlying rock without exposing a 

 substratum of bowlder clay. As this region bears evidence of intense glacia- 

 tion, it is to be expected that a bowlder clay should occur there also, if the 

 deposit lower down stream is directly of glacial origin. 



The deposition of bowlder clay by northern rivers is referred to on page 

 120 of this paper, where the agency of ice in modifying river deposits is 

 discussed. 



Direction of Ice Movement. — It has been determined by Dawson that the 

 main direction of ice movement in the upper Yukon region was about N. 

 8° W.* Local deflections conforming to the trend of the larger valleys have 

 been observed. 



During my journey from Lake Lebarge to the summit of Chilkoot pass 

 abundant opportunity was offered to verify Dawson's conclusions. On cross- 

 ing Chilkoot pass, and subsequently while traversing Lynn canal and the 

 " Inland Passage " south of Juneau, the general direction of former ice move- 



*Rep. Yukon District, loc. oit., 1887, p. 159b. 



