STRUCTURE 337 



thrust, and it is important to verify or disprove it by more extended 

 observations. So far as detailed topographic data are available south 

 of the 49th parallel, they show that the peneplain must rise mate- 

 rially toward the mountains to cross Saint Mary valley at a height suffi- 

 cient to meet the thrust surface, and this being so, the eastward slope of 

 the plain and the westward dip of the thrust would occupy anticlinal 

 attitudes, one to the other. It would follow also that the peneplain 

 must be warped, as is the thrust surface, since the elevations possibly 

 common to both are unlike in Flattop, Yellow, and Chief mountains. 

 In general, however, it is true from Divide mountain to Waterton lake 

 that the peneplain may be seen constantly to run into the foot of the 

 cliffs which mark the base of the known Algonkian. Before the over- 

 thrust was worked out, the writer observed this peculiar position of the 

 peneplain as one bearing on the physiographic history and presenting 

 difficulties. These difficulties lay in the problem of the relative ages of 

 the topographic features of the plains and those of the mountains. Flat- 

 top presents some interesting facts in this connection. 



The summit of Flattop is broad, gently sloping, long past maturity in 

 topographic phase. It bears no erratics, striae, or other signs of glacia- 

 tion. The rocks of the summit are quartzitic argillites, which are ex- 

 ceedingly resistant to erosion as compared with any rocks on the Plains. 

 The topography of the summit is unsympathetic to its environment and 

 it lies 1,200 to 1,800 feet above the position of the peneplain if the latter 

 be extended to the base of the cliffs. It follows that this past mature 

 topography on hard rocks could not have developed in its present alti- 

 tude above the plains. Either the surface of the Plains was higher and 

 has been lowered by erosion, or the summit of Flattop was lower and 

 has been elevated by thrust. 



If they formerly presented a topographic surface near the level of 

 Flattop's summit hills, the Plains have been degraded 1,200 feet, while 

 Flattop survived as a residual height. The Blackfoot plain on the Cre- 

 taceous rocks is so extensive and so completely planed as to indicate 

 a long epoch of erosion, and it seems improbable that Flattop could 

 have retained any part of its ancient summit hills, were they indeed 

 relatively so old. 



On the other hand, the summit of Flattop is of that topographic form 

 which would be reached by the harder rocks during the development of 

 the Blackfoot plain on the softer ones. The mass of Flattop rests upon 

 the inclined thrust plane, on which it has been pushed forward at least 

 7 miles. These relations strongly suggest that the summit of Flattop, 

 once nearly as low as the peneplain, has been pushed upward as well as 

 forward on the incline. 



L— Bum,. Geol. Soe. Am., Vol, 13, 1901 



