67 



tributary streams entering this valley have 

 hanging valleys, and in general the effects of 

 valley glaciation are here so pronounced as to 

 make it an ideal region in which to study these 

 phenomena. [17, p.p. 11—23]. 



The terraces seen along Lake Bennett at 

 various elevations possess considerable interest, 

 particularly as similar terraces characterize 

 almost all the main valleys not only in Central 

 Yukon but in adjoining portions of British 

 Columbia and Alaska. They are well devel- 

 oped along Lewes river below Whitehorse, along 

 Lake Laberge, along Nordenskiold river, and 

 elsewhere. A number of theories have been 

 advanced at different times to explain the origin 

 of these terraces, but the majority of such ex- 

 planations fail when all the known facts and 

 the extent of the region throughout which the 

 terraces occur, are considered. The writer has 

 investigated these in different districts in the 

 Yukon [17, pp. 21-23.] and Northern British 

 Columbia [18, see section on "Terraces"] and 

 believes them to be all lake terraces formed in 

 post-Glacial times, owing their origin to a tem- 

 porary damming of Yukon river near its mouth, 

 possibly by ice, and a consequent brief flooding 

 of the entire river system. 



66-7 m. Caribou — Altitude 2,171 ft. (660 m.) At 



106-7 km. Caribou the train crosses, on a swing bridge, a 

 narrow stream of water connecting Lake 

 Bennett and Nares lake. Thence for over 30 

 miles (48 km.) the railway follows a wide 

 northerly-trending depression, the floor of which 

 is deeply covered with glacial accumulations 

 which are in places hundreds of feet in thickness. 

 Everywhere the valley bottom is characterized 

 by kettle holes and morainal deposits, and 

 has the general appearance of still being almost 

 as the ice left it. About 15 miles (24 km.) north 

 of Caribou, a lake which was originally about 3 

 miles (4-8 km.) long was partly drained during 

 the construction of the railway, and there 

 splendid sections of the silts, in places exceed- 



34883— 5i 



