1905] WHIT FORD—FORESTS OF FLATHEAD VALLEY 109 
was probably thrown across the mouth of the Swan valley, and 
thus its connection with the main lake was cut off. A gap in the 
Mission Range offered a favorable place for the water thus cut off 
to make its escape. This outlet, of course, marked the beginning 
of that portion of the Big Fork River known as the “rapids” (map). 
Just as the present condition of the drainage system of Flathead 
valley is only a stage in the history of the destruction of the former 
enlarged Flathead Lake, so the existing drainage system of the Swan 
valley is only a stage in the history of the destruction of the former 
more extended Swan Lake. This system consists of Swan Lake; 
its inlet, Swan River; and its outlet, Big Fork River. Swan Lake 
is a body of water some twelve miles long and, except at its upper 
end, very narrow. It passes almost imperceptibly into the Big Fork 
River. This is a meandering stream which with its branches drains 
the valley and the mountains lying to the east and west of it. With 
the exception of small rapids here and there, and that part of it known 
as the “rapids,” the river is a rather sluggish stream. The valley 
(jig. 8) is approximately 945™ above the level of the sea. The Swan 
Range of the Kootenai Mountains rises abruptly from the east side 
of the valley to an altitude of from 1800 to 2200", and the Mission 
Mountains border the valley on the west. 
Such, in brief, are the main features of the physiography of the 
Flathead valley and its arm, the Swan valley. More detailed 
peculiarities will be noted in connection with the discussion of the 
edaphic formations; for, as will be shown later, there is an intimate 
relation existing between the destruction of the two lakes and the 
development of the plant formations that are found in the valleys. 
GEOLOGY. 
Flathead valley is not, as one would suppose, an erosion valley, 
but according to Witu1s’ it is due to a fault resulting in the down- 
throw of the region of the valley and an uplift of the region of the 
present Rocky Mountains. The northern Rocky Mountains con- 
sist of limestones, quartzites, and siliceous argillites about 2 750™ 
thick. During pre-Cambrian times the whole was under water. 
At i ts sata: of the Cambrian age an uplift brought it above 
» Bartrey, Structure of the front range, northern Bock? areata: 
Mota isice N. S. 15:86-87. 1902. 
