REPORT OF THE CHIEF ASTRONOMER 607 



SESSIONAL PAPER No. 25a 



structure will determine the former; studies of stratigraphy in relation to 

 sculpture will evaluate the amount by which erosion has reduced altitudes 

 relatively on the several rock types — argillite, limestone, quartzite. and 

 diorite. The determinations may be checked on some surviving areas of 

 ancient relief. When existing profiles have been raised or lowered in 

 accordance with these values, there will result a surface, which, in the 

 writer's judgment, will closely correspond with the peneplain over Galton 

 range. The conclusion involves elements which the eye cannot rightly 

 estimate in the field and for which precise data are not at hand. For 

 this reason the writer is disinclined definitely to place the peneplain rela- 

 tively to the heights of the Front ranges; but, recognizing the insignificant 

 extent of summit areas, or of shoulders that might support modified monad- 

 nocks, he thinks it may be located on top of the highest peaks rather than 

 below them.'* 



Combining the conceptions which are embodied in the quotation with 

 others contained in the body of Willis's paper, we may tabulate his hypothesis 

 as to the origin of the existing relief of the Front ranges, as follows : — 



1. The ' Algonkian ' strata were reduced to a peneplain in early Creta- 

 ceous time. This old erosion surface subsided beneath the Benton sea, which 

 extended as far west as about the longitude of Waterton lake. 



2. During Dakota and Benton time there was a very gentle and broad 

 upwarp of the Front ranges area, accompanied by sedimentation in a sea which 

 covered only the eastern part of the belt now occupied by the Lewis range. 



3. At the close of the Laramie (presumably at the time of the general 

 Laramide revolution) there was a single upwarp of the ' Algonkian ' and over- 

 lying Cretaceous beds, forming an unsymmetric fold with steeper dip on the 

 east. 



4. During the early Tertiary a long period of crustal repose during which 

 the upturned rocks were all more or less perfectly planed and the Blackfoot 

 erosion cycle completed. The peneplain was most perfect on the soft Cretaceous 

 rocks, but there was probably ' low, hilly, post-mature relief on the Algonkian 

 [Lewis series] rocks.' 



5. In the mid-Tertiary the great Lewis overthrust took place, whereby the 

 greatly eroded ' Algonkian ' block of the Front ranges and the equally broad 

 mass of the Galton-MacDonald group were uplifted. 



6. Apart from local normal faulting, the subsequent history of the region 

 has consisted in steady erosion, leading to mature mountain topography. 



In passing, it may be noted that the evidence of the earlier Mesozoic pene- 

 plain on which the Dakota and later Cretaceous beds were deposited, is not 

 made clear. It would seem probable that during the Mesozoic, this part of the 

 Cordillera was never far above sealevel. Most of the Mississippian limestone 

 formation is still preserved in the Crowsnest district only fifty miles to the 

 northward on the strike of the range. To the southeast its equivalent is like- 

 wise preserved beneath the Cretaceous beds of the Belt mountains. We have 



*B. Willis, Bull. Geol. Soc, America, Vol. 13, 1902, pp. 344-349. 



