WASHED GRAVEL PLAINS AND VASHON DRIFT. SW 
Duwamish valley, and the Cascade glacier from the east. It was trav- 
ersed by streams between the glaciers, and terraces of erosion are devel- 
oped on it. The limited areas in the interior of Des Moines island de- 
veloped between the tavo tongues of the Vashon glacier, one of which 
shrank back into the Duwamish valley and the other into Admiralty 
inlet. In each case the peculiar type of delta plain is bounded by azone 
of level plains which are not terraced and which mark the unsubmerged 
outwash plain beyond of a morainic zone. The conditions of extreme 
expansion and of initial retreat appear to have been followed by an epoch 
during which the glacier stayed for a considerable interval at a stage 
which overflowed the hollows and produced marginal accumulations 
upon the edges of the plateaus. 
Vashon drift, marginal featwres.—The components of the Vashon drift 
are round pebbles, sand, and loam. Compact clay is not a common phase 
of it. Angular and striated stones are in many places rare. The constit- 
uents are confusedly mingled over wide areas, but frequently they are 
sorted and even stratified. The roundness of nearly all pebbles and boul- 
ders suggests prolonged transportation in rivers. The structureless dis- 
tribution with local bedding indicates deposition from ice with aid of 
glacial streams. The marginal zones are composed of essentially the same 
materials’as the ground moraine, with the addition of large erraties, 
which are not numerous. The pebbles are throughout prevailingly of 
granite, of a variety which forms the mass of the northern Cascades. The 
sranite boulders are sometimes exfoliated and crumble, but the feldspars 
are not materially decomposed. ‘This fact serves to distinguish the 
Vashon gravels from others, which will be called the Orting gravels, in 
which the granite boulders can frequently be cut like stiff clay. 
The topographic configuration of the Vashon drift varies from smooth 
plains to a relief which exceeds a hundred feet (80 meters). Inthe fol 
lowing description of their distribution the marginal zones are distin- 
euished from the till by characteristic topography rather than by differ- 
ences of material or of internal structure. The distinction which was 
made in the course of field work is sustained in mapping by the rela- 
tive arrangement of the different types. Areas of undulating surfaces 
of gravelly loam, which are classified as till, are bordered by zones 
which are marked by ridged and hummocky relief. The gravel ridges and 
the irregular hills and kettleholes decrease in magnitude to the farther: 
limit of the outer zone, and there give place to smooth plains which be- 
long to the class of outwash plains or to terraced plains of washed gravel 
of the Steilacoom type. These relations are characteristic of formations 
marginal tothe ice. They define the direction in which it faced, and thus 
