SUMMARY. 155 
The glaciation of Puget Sound basin was the confluence of ice-streams 
from three sources, namely: (1) A northern ice-sheet which flowed over 
the San- Juan archipelago and westward through the straits of Fuca, but 
sent a massive glacier southward into the depression between the Cas- 
eades and Olympics; (2) the Cascade range in Washington, and (3) the 
Olympic range. These are named in the order of magnitude, the north- 
ern ice-mass having been much the greatest. 
The name Vashon is given to the till distributed by the northern gla- 
cier during the latest glacial epoch, and the term is extended to the epoch. 
Along the Duwamish valley lateral moraines prove that the valley 
antedated the latest epoch of glaciation and is not a channel of post- 
Pleistocene erosion. Similar evidence extends the inference to certain 
other valleys. : 
Deltas on the slopes to the Duwamish valley and others at higher 
level show that the Vashon ice served as a dam at various stages of its 
retreat and ponded waters in topographic embayments. 
Various topographic features, terraces, pitted plains, sterile washed 
plains, and channels are attributed to the vicissitudes of streams during 
glacial retreat and to the interaction of ice tongues and rivers. 
The Vashon drift is prevailingly of sandy loam and coarse rounded 
‘gravel. Its character and the topographic forms to which it gives rise 
prove that it was distributed extensively by glacial and subglacial 
streams. 
The topographic effect of the Vashon ice advance and retreat was to 
build up the plateaus particularly about their margins, to establish in- 
ward-flowing drainage systems, and to narrow the major hollows. The 
effect was to build up previously existing major features. The glacier 
did not obliterate the valleys under an even mass of drift. 
A till, called Osceola, which is characterized by rocks from the Cas- 
cades and their foothills, is recognized as distinct from the northern 
Vashon till. Its relative distribution and its relation in time to the Va- 
shon till are described. The Cascade glacier spread this till before the 
ice-sheets became confluent, and osars of Vashon drift developed on it. 
Certain coarse gravel beds which underlie the latest glacial formations 
are identified as being stream deposits with ice carried boulders. ‘They 
are correlated with the advance of the glaciers of the Vashon epoch. 
Though local, they typify deposits which may probably occur elsewhere. 
