450 /. HARLEN BRETZ 



invasion modified an older drift topography to that now existing. 

 As to which of the explanations is correct, this paper is not con- 

 cerned. For present purposes it is sufficient to show that, with 

 ablation of the Vashon ice sheet, the topography essentially as it 

 now is was exposed. 



The depression occupied by Puget Sound is inclosed on the west 

 by the Olympic Mountains, on the south by a low gravel plain 

 between the Sound and the Chehalis River, and on the east by the 

 Cascade Mountains. At the maximum of Vashon glaciation, the ice 

 sheet filled the entire depression, extending south of Olympia at the"* 

 most southern tip of Puget Sound. Outwash at this maximum stage 

 is largely responsible for the extensive gravel plain constituting the 

 Chehalis-Sound divide. 



When retreat of the front of the Vashon ice sheet began and 

 exposure of the depression of Puget Sound progressed northward, 

 water accumulated at the ice front to a depth sufficient to escape 

 across the gravel plain southward to the Chehalis River. The 

 meridional orientation of hills and valleys exposed by ice retreat, 

 together with the fact that the ridges were higher than the outwash 

 plain across which drainage escaped, caused the ponded water to 

 accumulate in long arms and inlets quite like those of the Sound 

 today. Troughs without opening southward and closed by ice at 

 the north would contain independent water-bodies whose levels were 

 controlled by the lowest place in their basin rims. Troughs with 

 openings lower than the Chehalis-Sound divide would become part 

 of a complexly branched lake, whose level was controlled by the 

 lowest altitude of that divide. 



How far north did the retreating fingered ice front serve to hold 

 up the increasing glacial waters to the discharge-way across to the 

 Chehalis River ? Postulating an uncrevassed ice sheet of sufficient 

 thickness, the dam should have been effective during retreat through- 

 out the entire length of the Sound. Denying this postulate, sub- 

 glacial drainage of the lake seaward (northward) might have occurred 

 early in ice retreat, in the manner described by Russell for lakes in 

 the Malaspina Glacier at the foot of the Chaix Hills. ^ Investigation 



I I. C. Russell, "Second Expedition to Mount Saint Elias," Thirteenth Annual 

 Report, U.S. Geological Survey, Part II. Suggested by Willis in personal communica- 

 tion to writer. 



