GLACIAL LAKES OF PUGET SOUND 455 



nel of Snoqualmie River into the Sammamish Valley. It is a gravel 

 plateau, one square mile in area, rising abruptly, along its western 

 and northern margins, from the lake valley bottom. Its summit 

 profile is made up of four levels, 120, 130, 140, and 160 feet A.T. 

 Excavations on the lakeward face show the entire height of about 

 70 feet to be composed of stream gravels, and in at least one case to 

 have foreset bed structure. The terraced surface clearly records 

 dissection of higher levels in the development of lower ones, and 

 proves their development in a lowering water-body. 



The relation of the highest level of the Redmond delta to the 

 York channel has been noted. For the remaining levels, 140, 130, 

 and 120 feet A.T., the present outlet past Bothel must have served. 

 The valley here is one of the few till troughs of Vashon or earlier 

 date, whose orientation is other than meridional. It must have 

 presented so low an escapeway on its exposure that it cannot account 

 for the three lower levels of the Redmond delta. Their explanation 

 must be sought elsewhere, and will be discussed under the following 

 heading. 



GLACIAL LAKE OF PUGET SOUND — -LAKE RUSSELL 



It is now time to examine the records of the master lake of Puget 

 Sound, whose level was determined by the Chehalis-Sound divide, 

 and which in turn controlled the descending levels of several, if not. 

 all, of the minor lakes of the region. 



This lake began its existence when the withdrawal of the Vashon 

 ice sheet first exposed surfaces lower than the great gravel outwash 

 plains between the Sound and the Chehalis Valley. Across these 

 plains all escaping water from the Vashon glacier must have flowed 

 as long as the ice remained a dam at the north. As already noted, 

 there is the possibility of escape of drainage northward through 

 englacial or subglacial tunnels at a level lower than the Chehalis- 

 Sound divide, before the clearing of the northern portion of the 

 Sound occurred. The northern limit of this great lake thus is at 

 present quite indefinite. 



The outlet for all water escaping southward from fresh-water bodies 

 in Puget Sound lies southwest of Olympia, through the col between 

 the Sound and the Chehalis River, with an elevation of 120 feet A.T. 



