92 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



fault-surface of the Lewis thrust.'* For his discussion of this and related points 

 the reader is referred to his paper. 



Willis also gives a detailed account of the interesting structural effects 

 wrought in the overriding block, particularly as illustrated in Chief mountain. 

 Similar evidences of the mighty force involved were observed by the present 

 writer on the Canadian side of the Boundary. As in the case of the Altyn 

 strata on Chief mountain the Appekunny beds north of Pass creek were seen 

 to be separated by flat-lying, heavily slickened surfaces, showing the existence 

 of minor thrusts with movement from west to east. At Chief mountain the 

 writer had opportunity of seeing the truly spectacular effects on the overriden 

 Benton shales. 



Willis has formulated a hypothesis according to which the existing structure 

 of the Clarke and Lewis ranges is attributed to two distinct periods of orogenic 

 movement. A summary of the hypothesis as it relates to the dating of the Lewis 

 thrust may be given in his own words : — 



' Along the eastern base of the Rocky mountains in general the facts 

 of structure express the action of a compressive stress, the Cretaceous and 

 older strata being folded. The post-Cretaceous effects are commonly attri- 

 buted to a single episode of compression; in what follows they are assigned 

 to two episodes, at least for the particular district under discussion. 



' The first episode of compression began at some date not closely 

 determinable, but which may be placed not earlier than Laramie time, 

 nor later than early Tertiary. It is possible that flexure went on during 

 Laramie deposition. It is also possible that it did not begin till after that 

 deposition was completed. The distinction is not important to the present 

 thesis. Flexure in its early stages was an effect involving relatively great 

 stress, as the nearly flat Algonkian strata were exceedingly inflexible. It 

 is probable that folds developed slowly. As the Laramie sea was shallow 

 and was succeeded by emergence of the area, the anticlines were subject 

 to erosion, whether they developed earlier or later, and the synclines received 

 their waste either as sediments beneath marine waters or in estuaries or in 

 lakes or as valley deposits. 



' The effect which for a time satisfied the compressive stress was one 

 of moderate folding. The succeeding condition was one of quiescence and 

 it endured long enough for the planation of Cretaceous rocks to the 

 Blackfoot peneplain. The name Blackfoot may be extended to the topo- 

 graphic cycle ending in the development of the plain. The Blackfoot 

 cycle cannot be accurately dated by any evidence now available. It was 

 post-Laramie and probably earlier than the orogenic movements which, 

 in Montana, gave rise to ranges and lake basins. The latter having yielded 

 Miocene vertebrates, the movement may be placed in mid-Tertiary. That 

 it was preceded by the Blackfoot cycle is an inference based on general 

 observations of an extensive peneplain over the summits of the Rockies 



• B. Willis, Bull. Geol. Soc. America, Vol. 13, 1902, p. 333. 



