THE EXTINCT LAKE YUKON. 147 



Lindemann, either the terraces or the sediments of the old lake are con- 

 stantly in sight. The lake probably extended far up the Yukon above the 

 mouth of the Lewes, and perhaps occupied the valley about Teslin lake ; but 

 this region is as yet unexplored. It occupied the valley in which Lake 

 Lebarge is situated, and also filled Ogilvie and Richthofen valleys which 

 open from it on the west. It also extended some distance up the valley of 

 Tahk-heena river. Above Lake Lebarge it formed an extremely irregular 

 water body, which filled the valleys now occupied in part by lakes Marsh, 

 Tagish, Bennett, and Lindemann. An exteusion eastward into the long, 

 narrow valley of Atlin lake is suspected, but has not been proven by obser- 

 vation. 



As will be seen from this brief description, Lake Yukon was extremely 

 irregular. It occupied a number of long, narrow valleys which chanced to 

 be connected and so situated as to be flooded by a single lake, having 

 a depth in the valley of Lake Lebarge of between five and six hundred feet. 



From north to south, Lake Yukon was about 150 miles long. Its width 

 near Miles canon and in the valley of Lake Marsh was about teu miles. In 

 other valleys it was much narrower, and even at its highest stage must have 

 appeared as a broad, placid river. Its extent, as well as the date of its ex- 

 istence places it among the more important lakes which were formed at vari- 

 ous localities on this continent during or immediately following the glacial 

 epoch. Of these, the best known at present are lakes Agassiz, Bonneville, 

 and Lahontan. 



Depth as shown by Terraces. — The highest of the horizontal water lines 

 above Lake Lebarge has been estimated by Dawson* to have an elevation 

 of 400 feet above the surface of the existing lake. My own estimates make 

 its elevation about 150 or 200 feet higher. The elevation of the surface of 

 Lake Yukon above the sea during its maximum expansion must, therefore, 

 have been between 2,500 and 2,700 feet ; the elevation of Lake Bennett being 

 taken at 2,150 as determined by Dawson. 



Sediments. — The fine, light-colored, horizontally stratified sediments of 

 Lake Yukon are well exposed in the steep river bluffs and along the lake 

 shores, all the way from near the mouth of Little Salmon river to Lake 

 Bennett. Fine exposures fully two hundred feet thick are to be seen along 

 the Lewes between lakes Lebarge and Marsh. At Miles canon the lake 

 beds rest on a floor of lava which is now being cut by the stream. The 

 channel occupied by the river previous to the existence of the old lake was 

 re-occupied only in part after the lake was drained and a new channel exca- 

 vated. There is here a fine example of superimposed drainage. 



Origin of the Lake. — The position of the outlet of Lake Yukon is as yet 

 uncertain. What held its waters in check also remains to be determined. 



* Report on the Yukon District, loc. cit., p. 159b. 

 XX— Bui.i,. Geoi.. Soc. Am., Vol. 1, 1889. 



