VASHON EPISODE AND ITS HISTORY. 141 
but they may have been accumulations along the margin of the ice, and 
thus belong to the class called kames. 
GEOLOGY OF THE PLEISTOCENE Hpocns 
GENERAL STATEMENT 
The preceding pages present the existing topographic aspects of the 
Puget Sound basin, and particularly those of the Tacoma quadrangle. 
The topography is so directly a result of geologic conditions that it in- 
volves a statement of the nature of the latest episode of glaciation; but 
the further description of glacial and interglacial formations belongs to 
the subject of geology. Therefore the facts and inferences of the Vashon 
episode are here summarized and followed by a description of the under- 
lying Pleistocene formations. The various deposits occur separately in 
different sections of the plateau slopes, and correlation from one exposure 
to another is not alwayssure. The principal sections are given in tabular 
form at the close of this chapter. The first (Section A) may be regarded 
as typical, as it includes all of the members found elsewhere. 
THE VASHON GLACIAL EPOCH 
Definition oF terms.—The Vashon episode has already been defined as 
covering that interval of Pleistocene time in which the glaciers most 
recently spread into the Puget Sound basin and there became a confluent 
ice-mass. The name Vashon, from Vashon island, is given specifically 
to the ice-tongue which then pushed southward from the highlands of 
theSan Juan archipelago ; but the Vashon episode also covers the expan- 
sion of the contemporaneous glaciers from the Cascades and mount 
- Rainier, of which one, the Osceola tongue, is known by its distinctive till. 
Summary of the Vashon history.—In consequence of the advance of the 
Vashon glacier, there was carried from the north a mass of gravel, sand, 
and loam, which was spread over wide areas of the Puget Sound basin 
as the latest drift sheet. The glacier extended far into the southeastern 
arms of the sound, the extreme limit as yet recognized being a short dis- 
tance south of Carbonado and about 1,600 feet (488 meters) above the 
present sealevel. ‘The margin of the gravelly Vashon till is not marked 
by terminal moraines, but is characterized by osars. The deposit—in 
large part, at least—is of subglacial origin, modified by streams. Por- 
tions of it may be englacial or superglacial. Its limits are related to the 
topography of the region over which the ice stood. They do not neces- 
sarily correspond to an ice-front or the face of the glacier, and probably 
along certain sections the Vashon ice did not present a face. Along such 
sections—as, for example, the line of the Wilderness swamps—it was 
