REPORT OF TEE CHIEF ASTRONOMER 93 



SESSIONAL PAPER No. 25a 



of western Montana and Idaho, observations which leave no doubt in the 

 writers mind of the existence of such a peneplain, but which do not suffice 

 positively to identify it as the Blackfoot plain. On the probability of that 

 identification the Blackfoot cycle may be placed in early Tertiary time. 



' At the close of the Blackfoot cycle the topographic features of the 

 region under discussion were the peneplain on Cretaceous rocks and low 

 hilly, past-mature relief on Algonkian rocks, such as is now presented by 

 the summit hills of eastern Flattop 



' Among the effects of folding and erosion, at the close of the Black- 

 foot cycle was the exposure of the edges of some Algonkian strata as out- 

 crops; being gently inclined westward, these beds had probably wide extent 

 underground. They were relatively stiff and lay with one edge free. Under 

 these conditions, supposing that a compressive stress again became effec- 

 tive, a part at least of the Algonkian beds were so placed that they met but 

 slight resistance in their tendency to yield by moving forward. So far as 

 they were unopposed, or not sufficiently opposed to check and fold them, 

 they did ride forward. That part which was thus overthrust separated 

 from that which was not in general along bedding planes near the base of a 

 particularly rigid stratum, such as the Altyn limestone. The Siyeh lime- 

 stone, the Carboniferous limestone, or other stiff formation may else- 

 where be found to have determined the thrust surface within the old rocks.'* 



Willis then concludes this postulated history by noting the geological date 

 deduced for the Lewis thrust and directly associated movements. He also 

 briefly states the alternative view that the folding and thrusting were products 

 of a single period of deformation. He writes : 



' On the hypothesis of a single episode of compression, from which 

 resulted all the phenomena of folding and thrusting in Cretaceous and 

 Algonkian rocks in the district, the Lewis thrust and the associated struc- 

 ture? must be assigned to a date closely following the Laramie deposition. 

 The growth of the Front ranges and the development of the Blackfoot 

 plain must be placed later, and the expression of the Lewis thrust must be 

 considered subordinate at the surface to these later effects of orogeny 

 and erosion. 



' On the other hand, on the hypothesis of two episodes of compression, 

 separated by the Blackfoot cycle, the Lewis thrust must result from the 

 second episode, and falls probably in mid-Tertiary. Its orogenic effects 

 are then dominant in the Front ranges, and the physiographic history is 

 to be read in terms of structure as well as of erosion. 



' It is concluded that the date of the Lewis thrust may be placed in 

 either late Cretaceous or mid-Tertiary time, and the principal criteria for 

 determining which date is correct are to be found in the relations of struc- 

 ture to physiography. 'f 



* B. Willis, Bull. Geol. Soc. America, Vol. 13, 1902, p. 339. 

 t Ibid, p. 343. 



