FORMATIONS OF THE VASHON EPOCH. 143 
earry gravel only over the plateau surfaces, As the ice melted back the 
interiors of the plateaus were first laid bare. ‘They were scoured by the 
escaping waters, while yet osars and lodge moraines formed about the 
margins. When the ice sank into the hollows the internal drainage of 
the plateau areas persisted, finding such outlets as are indicated by Fen- 
nel creek and Big Soos creek. The resultant effect on the topography 
of the region was to build up the plateau margins and to establish an 
inward flowing system of drainage in each plateau area. 
The effect of Vashon glaciation in narrowing and filling the major hol- 
lows is obscured beneath the more recent sediments and the water bodies 
of the sounds. Invading the region, the ice first occupied the hollows. 
Melting away, it lingered longer in them than on the plateaus. As the 
lowest part of the glacier, the ice in the hollows was probably very densely 
charged with gravel. The depth of the hollows at the present time rep- 
resents the thickness of ice, less the amount of later sediment, beneath 
which hes the gravel which the ice contained. 
The formations which in addition to the Vashon drift constitute the 
récord of the Vashon epoch may now be described. 
Osceola till —The name Osceola is here given to a sheet of till which 
covers the plateau between Green and White rivers and extends south- 
west beyond White river about the head of Fennel creek. It is covered 
by the osars around lake Tapps and by the delta sands on the south. 
The Osceola till consists of fine silt with numerous angular stones. 
The silt is a glacial meal such as White river carries at the present time. 
The stones are prevailingly, if not almost exclusively, fragments of Ter- 
tiary or later volcanic rocks. They are coated with a lmy deposit and 
with the clinging silt. The till forms a hardpan upon which the growth 
is that of swamps. Peat bogs are numerous over its area. 
This till was spread from the east directly by a glacier descending from 
 theCascades. It is easily distinguished from the Vashon till by its com- 
pact silty constitution and numerous stones of volcanic rocks. It also 
contains blocks of Kocene sandstone and shale. Water, which played 
so important a part in distributing the Vashon till, was a subordinate 
agent in the formation of the Osceola till. 
The Vashon osars about lake Tapps appear to rest on the Osceola till, 
and consequently to be younger. The two formations, nevertheless, prob- 
ably belong to one epoch of glaciation. The source of the Vashon ice 
was remote, that of the Osceola ice was comparatively near. The Vashon 
glacier advanced first along the deep hollow of Duwamish valley, the 
Osceola glacier descended from the foothills on to the plateau. From 
these relations it follows that the Osceola till may have been spread 
while the Vashon glacier was still confined in the hollow or even distant, 
XXI—Burt. Guox. Soc. Am., Vou. 9, 1897 
