Geolof/i/ of I'lKjet Sodiid Basin. — Kimball. 311 
of the drift terrane above soundings may be taken at some 
1,350 feet. 
Well exposed sections of the drift, as seen in numerous ver- 
tical scarps, often exhibit more than a single mode of structure. 
Suffice it to notice, at ditferent relative levels, unequal tiiick- 
nesses of horizontal silts, generally iniconforniabl}' overlying 
plunging and lenticular sands and gravels. Laminated s-ilts, 
over 100 feet thick, are perfectly exhibited in a vertical l)luf!' 
on Duwaniish (P^iliott) bay, at West Seattle, and also on the 
opposite side, in many street cuts at Seattle. Such deposits, 
some 25 feet in thickness, crown the Puyallup divide in the 
outskirts of Tacoraa, and there constitute the locus of angular 
erratic blocks up to several tons in weight. Smaller erratics, 
likewise angular, are also sparingly strewn through the Du- 
waniish bay silts. Both occurrences point to former higher 
delta levels of glacial I'ivers — iiamely, the Puyallup atTacoma 
and the Duwaniish at Seattle, and to distribution of erratics 
by bergs.* 
*In sinking at intervals through several hundred feet of the alluvial 
column at Tacoma, numerous facts were developed which go to .=ho\v 
moditicatiou of the drift in local circumstances, mainly through the 
agency of underground cin.-ulatiou of water under conditions of a copi- 
ous supply, free percolation and a hydrostatic head. I refer in particu- 
lar to downward filtration of finer detrital materia! through the coarser, 
resulting in assortment of miscellaneous detritus into bedded or lentic- 
ular gravels, sands and clays, in the order named, usually upon a floor 
of impervious hardpan. Gravel beds become coarser toward the bottom. 
One bed ob.served at drainage level, about tiOO feet above tide, giad nates 
into large cobble stones. All such occurrences throughout the region 
seem to be near the same relative level — that is, never far from immedi- 
ate water level. The upper gravels are fine and generally more or less 
cemented by ferric hydiate. Gravels cemented by clay occur only be- 
low the water-bearing horizon — no mechanical separation, as above indi 
cated, having taken place under conditions of quiescent saturation, or, 
as it appears, below the level of emission. 
Another interesting ttfect of the same remarkable local conditions 
may be mentioned as follows: At the level of the principal water-bear- 
ing horizon, about 2.35 feet above mean low tide, undermining takes 
place, followed by rapid erosion of ravines, by which the plateau is dis- 
sected. Above that level they are ordinarily dry: while below, the ac- 
cumulated flow has its own cutting power. Three ravines, known as 
East, Middle, anfl West gulches, have been sculptured from the drift 
terrane down to the remarkable depth of 100 feet to tide level, in the 
short distance of one and one-half miles their extreme length. Narrow 
watersheds, such as the intervening belts of plateau, crowned as they 
are with impervious silts, cannot be the immediate source of the several 
flows. The supply of water under a hydrostatic fiead, on the contrary, 
is obviously referable to hydn^grapliic basins at a considerably higher 
elevation within the drift sheet, and to sources near its upper edge. The 
nearest approach (if the Puyallup river is less than two miles. Similar 
ravines are characteristic of the shore of Commencement bay all the 
way to Point Defiance. 
