THE NORTHERN PACIFIC ROUTE. 
135 
side of the river, is Fort Missoula, one of the principal military posts 
in the mountain region. 
and also off the railway in the Bitterroot 
Valley and across the divide north of 
Ravalli were undoubtedly formed by a 
continuous body of water that at some 
recent geologic date occupied these val- 
leys. On account of the excellent devel- 
opment of the beaches of this lake at Mis- 
souls. it has been named Lake Missoula. 
(See map on sheet 19, p. 144.) 
Lake Missoula must have occupied the 
valley at a very recent date, for the faint 
would have been entirely 
re 
ever, although it was geologically recent, 
it existed many, many years ago, probably 
long before the eect to roam over 
these hills and m 
Lakes are ‘ieee ease and are due 
to some interference with the normal 
d + 4 nf} | 4 
+ of tha 
region, What then occurred in this 
region to change the drainage and to cause 
the ponding of the streams at Missoula to a 
depth of 1,000 feet? The altitude of the 
highest beach line that has been observed 
is about 4,200 feet at Stevensville, in the 
Bitterroot Valley south of Missoula, and 
as the altitude of Missoula is about 3,200 
feet, the depth of the water must have 
been about 1,000 feet 
When the beaches are traced north- 
ward and westward, they are found to 
terminate just in front of the southern- 
thus show a definite relation to the ice 
front and as they seem to correspond in 
time with the glacial epoch, it seems alto- 
gether probable, if not certain, that Lake 
Missoula was due to the damming of Clark 
Fork by the ice. The great ea that 
swept down from Canada at this time are 
known to have occupied all the mountain 
pth e 
which : range from a few hundred to per- 
haps thousands of feet. One lobe of this 
mass of ice came down the Flathead Val- 
ley as far as the Northern Pacific Railway 
Dixon, and another down the broad 
valley from Bonners Ferry, on the Koo- 
not they were also filled with ice, but it 
seems probable that at least some of them 
afforded avenues for the southward flow 
of small tongues of ice nearly or quite to 
Clark Fork 
All the evidence points to the conclu- 
sion that the main valley of Clark Fork in 
the vicinity of Pend Oreille Lake was 
effectively blockaded by the ice, and that 
the low valleys to the north were shut off 
as avenues of escape for the waters of the 
upper valleys. Such a dam would nec- 
essarily be inconstant, allowing the depth 
of water to fluctuate considerably, and 
consequently many shore lines would be 
cut on the rocks; but none of them would 
y marked, as the water was not 
held long enough at ag one level to per- 
mit deep cutting. shifting of the 
positions of the 5 ice lobes would 
also tend to produce a — level of 
the outlet and a correspondi 
level of the surface of the water, As the 
grew thinner and thinner and 
shrank in a co nding 
at last the present outlet was opened and 
the water disap 
Although the gen neral 
Missoula is about as! sketched, a nae of 
fications: may be necessary when the > final 
history of the lake is written. The most 
difficult to harmonize with the thoary 
of the beach lines in the several valle eys. 
Thus at Stevensville, in the Bitterroot 
Valley, they extend up the valley wall to 
an altitude of 4 200 feet; north of Dixon 
to 3.950 
feet; at Plains they can be traced up 
altitude of 3,100 feet, but above ‘hed tral 
Sa oem 
Trout Creek they apparently : 
cease at 3, sol and on St. Regis River 
