142 B. WILLIS—DRIFT PHENOMENA OF PUGET SOUND. 
opposed by and became confluent with glaciers descending westward or 
northward. 
The maximum thickness of the confluent ice as yet determined was 
1,600 feet (488 meters) above the present sealevel, or 1,000 to 1,200 feet 
(300 to 400 meters) above the plateaus. As the ice filled the Duwamish 
valley, and without doubt also the hollows of Admiralty inlet and other 
sounds, its thickness along their courses must have been 1,600 feet (488 
meters) plus the depth of water, at least. Over the greatest depth of 
Admiralty inlet the ice was, therefore, not less than 2,500 feet (800 meters) 
thick, and if allowances be made for southward slope of its upper surface, 
it was probably more than 3,000 feet (900 meters) thick. 
That the ice occupied the Duwamish valley is snown by the occurrence 
of drift and lateral moraines along the slopes of the valley. From this 
fact follows the important inference that the hollow of the Duwamish 
valley is older than the last glacial advance. It is not a channel of post- 
glacial erosion. The relations of the homologous hollows of Admiralty 
and other inlets to the Duwamish valley is such that the inference neces- 
sarily extends to them also. Hence we may conclude that before the 
Vashon epoch the major features of Puget Sound physiography were 
similar to what they are now. The Vashon ice invasion modified but 
did not obliterate the major antecedent topographic features, and they 
still persist. 
The effect of the Vashon invasion on the topographic configuration of 
the region was constructive. The Vashon till is spread as a sheet, which 
is a general addition to the masses of the plateaus. It is of very unequal 
thickness. Observations of its extent on steep slopes vary from 25 to 200 
feet (8 to 60 meters) below the summits, but they are of little value as 
determinations of thickness, since they are liable to errors due to over- 
placement, landslides, erosion, and inequalities of the underlying surface. 
Wells which might yield more satisfactory results have not done so, as 
the base of the Vashon drift is not easily distinguished. Where the drift 
is heaped in osars and lodge moraines the thickness materially exceed 
the average. Taking these excesses into account, it is reasonable to 
assume that the volume is equal to that of a uniform sheet 65 feet (20 
meters) thick extended over the areas of the plateaus. The hollows are 
not included, as there is no equivalent basis for an estimate of the amount 
of till beneath the alluvium and water. 
As the thickness of the Vashon drift is abruptly variable, the distribu- 
tion of its greater additions to the antecedent relief may be observed. 
They are found to be marginal to the plateaus and to be highest near 
the outer rims. When the ice filled all the hollows and overflowed over 
the plateaus, subglacial streams could escape with current sufficient to 
