136 B. WILLIS—DRIFT PHENOMENA OF PUGET SOUND. 
The Steilacoom plains, which extend for many miles south and south- 
west from Tacoma, constitute the type of this formation. It was studied 
most carefully in 1896 by my associate, Mr Smith, and agreeing, I adopt 
the following views from his manuscript report: The deposits of the 
Steilacoom formation are of glacial origin and present a peculiar type 
of washed plains. They differ from the ordinary type of outwash plain 
in their wide extent, in the widespread distribution of coarse material, 
in the absence of gradation from coarse to fine material in going from the 
morainic zones outward, and in the marked horizontal terraces. ‘The 
superposition of horizontal terraces on uneven surfaces of morainic kettle- 
holes and ridges in such manner that the latter are more or less obscured 
indicates a sequence of conditions. The morainic features belong to an 
episode when the ice-front was for a brief period maintained along a 
given zone, whereas the terrace features are deltas formed in waters lying 
in front of the ice-face. The Steilacoom plains therefore represent an area 
from which the ice retreated, and which was submerged during the epi- 
sode of retreat. The deltas might have been formed in shallowing or deep- 
ening waters. Ifthe former had been the case the upper deltas should be 
more or less dissected and the lower should be well defined. The reverse ~ 
istrue. The lower deltas are somewhat masked by later deposits, whereas 
the upper deltas are sharp and complete. It therefore appears that dur- 
ing the development of the peculiar terraced surface the ponded waters 
were deepening, a result likely to occur during the retreat of the glacier 
where the topographic features afforded basins. 
Thus the Steilacoom plain is a zone of glacial retreat, the glacial de- 
posits proper being of low relief and for the most part covered or masked | 
by gravel deposits mostly of delta character. This hypothesis seems 
best to account for the great extent of the plains, the isolated kame-like 
areas, the mound and basin surfaces, the different terraces, the gravels 
which compose the subsoil, and the silt which veneers the gravel surface. 
Had the lake been a more permanent feature at any level a gradation in 
grain of the deposits might be found, and at some distance from the ice 
the silt deposit should be thick enough to furnish a good soil. 
. The areas in which these plains are best developed are the Steilacoom 
plains already referred to and a district from 6 to 8 miles (10 to 15 kilo- 
meters) east of Kent. Plains of similar character, but less conspicuously 
sterile, occur 3 miles (5 kilometers) north of Des Moines, and also in the 
wider southern portion of Des Moines island. In each case the topo- 
graphic type marks an area from which the confluent ice receded from 
the maximum expansion of the Vashon epoch. ‘The wide Steilacoom 
plains were overflowed by the southern lobe, The area east of Kent was 
intermediate between the Vashon ice-tongue, which overflowed from the 
