Photo by George M. Weister 
AN ICe;-WALL in the COLLIER GLACIER 
"A circular opening into which the water plunges to unknown depths with a muffled, ominous 
roar" 
raines, heaps of glacier detritus far down 
the mountain side. 
The Three Sisters are typical volcanic 
peaks. Only the south and middle peaks, 
however, have sufficiently resisted the 
destructive processes to exhibit the dis- 
tinctive cone profile of the volcano, and 
South Sister alone possesses still a shal- 
low crater in its top. North Sister is an 
elongated, jagged ridge of unstable lava, 
culminating in a massive pinnacle with 
almost vertical creviced sides, whose loo 
feet of height had, until 1910, so far as 
is known, successfully daunted the en- 
thusiasm of all aspirants but one, Mr. 
H. H. Prouty, of Portland, Oregon, who, 
unaided, made the ascent in August of 
that year. This same season a commit- 
tee of the Mazamas succeeded, by means 
of ropes, in placing the official record- 
box on the apex of the peak. 
The peaks themselves are composed of 
volcanic materials entirely, and the varie- 
ties of rock represented in their masses 
and covering large adjacent areas in 
every direction suggest a succession of 
eruptive periods, during different ones of 
which different rock species predomi- 
nated. 
In general keeping with the trend of 
the Cascade Range, , volcanic outflow 
seems to have proceeded from vents 
along a series of fractures in the earth's 
crust running north and south. Evi- 
dence shows that these eruptions have 
taken place at intervals throughout the 
long lapse of geologic time, from the 
Tertiary period almost to the present. 
The earlier lavas were prevailingly 
andesitic, while the more recent craters 
have discharged vast quantities of basalt 
and other markedly basic extrusives. 
Aside from the symmetrical outlines and 
crater of South Sister, additional evi- 
