604 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



GaUon-MacDonald Horst. — Between the Flathead and Gateway-Kootenay 

 grabens we have the compound horst which may with advantage be considered 

 as a physiographic unit, though it is topographically divisible into the Galton 

 and MacDonald mountain ranges. 



The relief is not so great as in the Front ranges. On the east the local 

 baselevel is given by the Flathead river at about 4,000 feet; on the west, by 

 the Kootenay at about 2,300 feet. The highest summit of the MacDonald range 

 in the Boundary belt is 7,724 feet high. The highest summit of the Galtons 

 in the belt is 7,930 feet. Most of the ridges in each range average about 7,000 

 feet in height. Comparatively few of. them are above tree-line, which is at 

 elevations, varying locally with the nature and exposure of the slopes, of from 

 7,300 to 7,700 feet. 



The quality of the topography is also contrasted with that of the Clarke 

 and Lewis ranges. Though the rock-formations belong to the same horizons 

 as there, forming simply a more westerly phase of the Rocky Mountain Geosyn- 

 clinal, the beds are distinctly less heterogeneous and seldom show the cliff and 

 talus form even where the beds lie flat. The Purcell Lava is almost the only 

 member which preserves the cliff-making property, though the massive Siyeh 

 formation tends to form specially steep slopes on its outcropping edges. The 

 mountains of each range make up a rather rugged assemblage of ridges crowned 

 by occasional low horns, but the true precipice is seldom seen. The reason for 

 this is double; partly due to the softening effects of the general forest-cover, 

 partly to the relative weakness of the Pleistocene local glaciation as compared 

 with that across the Flathead. Steep as many of the ridge slopes are, they are 

 those of graded profiles in a maturely dissected mountain-range. The grading 

 was pre-Glacial and the associated veneer of creeping rock-waste was largely 

 removed during the general glaciation of these ranges. 



Within the horst itself the relief is clearly due for the most part to normal 

 stream erosion, acting on a number of fault-blocks. The explanation of the 

 topography is, therefore, at hand, as in the case of the Front ranges, if we can 

 arrive at final conclusions as to the origin of the streams and their valleys. 

 Again we are baffled in reaching that desirable end, and most suggestions are 

 subject to doubt until much further field-work has been accomplished. It 

 would, however, seem probable that the narrow Boundary belt offers an average 

 sample of the topography for the whole mountain group and its indications are 

 not without value. We have already seen that the Flathead and Kootenay are 

 respectively located on fault-troughs and may, therefore, be classed as conse- 

 quent rivers; as they run parallel to the Cordilleran axis we may further 

 describe them as longitudinal consequents. The Wigwam river seems «to be 

 located on a master fault, though not on a well-defined graben. Analogous 

 relations are observed among some of the branches of the Wigwam in the 

 Boundary belt. (See map.) Most of the other creeks and canyons of 

 the group have no discernible relation to rock structures like faults or folds; 

 the larger transverse streams appear to represent consequents draining the 

 compound horst to east and west on the constructional fault slopes, but they 

 have gnawed well back into the now well dissected fault-blocks. The lack of 



