physiography 345 



reasonably attributed to difference of hardness of rocks. Limestones 

 and quartzites could not have maintained such relative altitude so near 

 a lowland in which shale and sandstone were reduced to a plain. The 

 later forms sculptured in the Blackfoot plain are apparently represented 

 by equivalent features in the Front ranges. When their correlation has 

 been worked out, remnants of a surface may be recognized as belonging 

 to the Blackfoot cycle in old age. They may be traced among high 

 shoulders of the peaks, which must then be considered monadnocks, or 

 they may be the tops of peaks. In the latter case the surface may ap- 

 pear closely to conform to the highest summits of the crests and to lie 

 above the structural valleys. The criteria which effect this alternative 

 can better be discussed when the structural factors shall have been 

 estimated. 



The relatively great altitude of the Lewis range, considered as a result 

 of mountain growth, might be attributed to a monoclinal flexure or a 

 normal fault ; but there is neither monoclinal flexure nor normal fault 

 between the Lewis range and the Plains. There is, however, a great 

 overthrust, and it is appropriate to consider the quantitative sufficiency 

 of the thrust to produce the difference of elevation. Eastern Flattop 

 mountain furnishes most satisfactorily the data for estimate. Its surface 

 is topographically old. It may represent the Blackfoot peneplain as far 

 as it developed on silicious argillite, or it may have suffered some erosion 

 in initial uplift immediately preceding the thrust movement. In the 

 latter case the surface which should be compared in altitude with the 

 Blackfoot plain lies somewhat above existing features. The highest hill 

 on Flattop, a low, rounded summit of bare rock, is 8,340 feet above 

 sea. It is reasonable to place the ancient surface not above 8,500 feet, 

 or not more than 3,500 feet above the Blackfoot plain, in Milk River 

 ridge. By reference to figure 4 it will be seen that the dip of the thrust 

 plane beneath Flattop is about 4 degrees, with a corresponding displace- 

 ment of 4 miles. The vertical component, or vertical throw, of the 

 thrust for this section is a little less than 1,500 feet. The observed dis- 

 placement of Flattop is 7 miles, leaving 3 miles not reckoned in the 

 above figures, and the dip is known to become materially steeper as the 

 thrust descends. If the average dip for 3 miles below the outcrop at 

 Swift Current falls be 7 degrees (or about that determined southwest of 

 Chief), the vertical component of the thrust would be a little more than 

 1,900 feet. The two estimates should be added, and their sum is 3,400 

 feet. In these figures the only one not checked by observation is the 

 dip of 7 degrees below the outcrop, but it is less than would be esti- 



LI— Bull. Geol. Soc. Am., Vol. 13, 1901 



