Geology of Fuget Sound Basin. — Kimhall. 317 
The longer axis of the sound, wliich is also the axis of the 
great drift terrane, conforms to the natural confluence of 
powerful streams which have wrought their beds to tide level. 
Such streams are the Skagit, Sks'komish, Stilagiiamish, Sno- 
homish, Duwamish (uniting the watei's of lakes Saniamish 
and Washington with those of Cedar and White rivers), the 
Pu^'allup, Nisqually and Skokomish. iVs compared with the 
gentle gradients of the lower parts of these sti-eams, much 
higher gradients at the close of the glacrial period may safely 
be assumed. These of course were in some measure of the 
elevation of the drift terrane. Regional elevation or post- 
glacial re-elevation, of which local evidence has been adduced, 
has likewise tended to augmented gradients. All gradients 
have eventually become distributed with the advance of flu- 
vial erosion. Below points of confluence of lateral rivers 
still deeper excoriation has been effected by flow of the united 
waters toward the strait of Fuca. Thus were profoundlj^ 
sculptured great confluent river channels, flrst, in measure of 
the elevation of the drift, and afterwards in increased meas- 
ure of regional elevation. Again, subsidence far below the 
sea level succeeded by only partial re-elevation has Anally 
served to keep open to the sea all but the more easterly or 
border channels, and to produce the present system of united 
inlets together known as Puget sound. Wherever consider- 
able streams unite with the sound, bays have been washing 
out ever since the principal post-Glacial subsidence, and estu- 
aries, primary and secondary deltas, and tide-flats forming 
under the precipitating action of a heavy tide. 
The more easterl}^ extinct channels referred to are lake 
Samamish and the river of the same name (also known as 
the Slough), connecting with lake Washington. These wa- 
ters constitute a single hydrographie system, the lower de- 
velopments of which have obviously been cut off from the 
sound by regional elevation subsc(|uent to fluvial erosion of 
these connected channels. 
At a certain stage of level in the course of the later series 
of oscillations, the lower channel corresj)onding to lake Wash- 
ington, doubtless opened into lake Union, and thence into the 
present estu.ary of Salmon bay. The ancient waterways are 
plainly indicated in the present topography. A minimum 
