THE GLACIAL LAKE MISSOULA 383 



continuous barrier exists, a partial one only being afforded by the 

 detached Cabinet, Flathead, and Galton ranges. Between the 

 latter and the continental divide the valley of the upper Flathead 

 River at the 49th parallel sinks slightly below the given plane, and 

 continues, as a depression (the Rocky Mountain trench of Daly),' 

 a great distance northwest into British Columbia. 



Between the Galton range and Cabinet Mountains is a considerable 

 area which, although partly occupied by the Flathead Mountains, 

 affords two broad passes through which the water would escape to 

 the Kootenai Valley. South of the Cabinet Mountains, the Clark 

 Fork Valley is depressed at the Idaho-Montana boundary 2,100 

 feet below the 4,200 level. At this point the valley is rather con- 

 stricted, its cross-section showing a width of three miles near the 

 river level, and about seven miles at 4,200 elevation. 



Of special interest to the problem at hand are three north-south 

 depressions that join the Clark Fork and Kootenai valleys. The 

 largest and most important of these is the Pack River or Kootenai 

 Pass through which the Great Northern Railway crosses from Bonners 

 Ferry on the Kootenai to Lake Pend d' Oreille. It is a rather broad, 

 deep valley whose highest point is about 150 feet above lake Pend 

 d' Oreille and is in reality, as shown by Calkins,^ a part of the Purcell 

 trench, a depression extending northward 200 miles beyond the 49th 

 parallel. 



The two smaller trenches cross the Cabinet Mountains east of 

 this one, the Bull Lake trench^ affording an easy pass with only 700 

 feet climb, between Smead on the Clark Fork and Troy on the 

 Kootenai, and, farther east, a depression crossing the same mountains 

 between Plains and Jennings. From the foregoing it appears that 

 at the present time a barrier of sufficient height across the depressions 

 just described would restrain a lake in the drainage basin of the 

 Clark Fork, whose waves would terrace the 4,200-foot contour. 



This would also be the case in Pleistocene time if the physiography 



I Reginald A. Daly, The Nomenclature of the North American Cordillera betzueen 

 the 47th and jjd Parallels oj Latitude, Ceog. Jour., XXVII, No. 6 (1906), 596-98. 



~ F. C. Calkins, "A Geological Reconnaissance in Northern Idaho, and North- 

 western Montana, Bull. U. S. Geol. Survey, No. 384 (1909), 16. 



3 Ihid., 15. 



