156 C. W. Hayes — Expedition through the Yukon District. 



a rapidly retreating glacier and the streams to which it gave 

 rise. These gravels are younger than the bowlder clay which 

 . they overlie and also younger than the silt of the river bluffs to- 

 ward the north. 



Among the most interesting deposits associated with the 

 second period of giaciation in the northwest are those forming 

 the river bluffs along the Teslin and other tributaries of the 

 Yukon. Bluffs are continuous throughout the whole length of 

 the Teslin river, increasing slightly in height from about 100 

 feet at the lake to 150 feet at the mouth, and frequently cut 

 into a number of terraces. The materials of which they consist 

 are light colored silts or fine sand interbedded with layers, one 

 to three inches in thickness, of tough bluish clay. The layers 

 of sand are often cross-bedded and contain sufficient clay to 

 give the material considerable tenacity. At some places inter- 

 mediate beds are highly contorted, while those above and beloM'' 

 are undisturbed. Although the deposit differs widely from the 

 true bowlder clay which it was seen to overlie, yet it contains 

 o'ccasional large angular bowlders, evidently brought to their 

 present position by floating ice. The bluffs are usually capped 

 by a bed of coarse gravel, ten feet or more in thickness, but 

 sharply separated from the underlying silt formation. More 

 rarely, layers of coarse sand and gravel a few feet thick occur, 

 interbedded with the silt, usually toward the top. 



This deposit undoubtedly belongs to the wide-spread '"' white 

 silt " formation which Dr Dawspn has described as occurring at 

 many localities in British Columbia and the upper Yukon basin. 

 He regards the white silt as a de230sit laid down in estuaries by 

 waters containing glacial mud supplied b}^ streams from the re- 

 treating or stationary ice front. The altitude of Ahklen is 2,500 

 feet, and hence the upper limit of the silt in the bluffs at the 

 lower end of the lake is about 2,600 feet. The upper limit of the 

 white silt, as observed by Dawson at various points in British 

 Columbia and the Yukon basin, is between 2,400 and 2,700 feet, 

 indicating a subsidence to that extent for a considerable period 

 toward the close of the second epoch of giaciation. During this 

 period of subsidence the present lake basin was doubtless occu- 

 pied by a lobe of the retreating glacier which prevented the 

 silting up of the portion of the valley so occupied. On its Avith- 

 drawal at the close of the stationary period, the lake was left 



