[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 117 



ranges of the Rocky mountains has been well summarized by Daly 

 (12, f ) as follows : 



L The "Algonkian" strata were reduced to a peneplain in early Cretaceous 

 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 overlying 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 explaining the Lewis thrust, it is clear that Willis considered 

 it to be of the type to which he had long before given the term "erosion 

 thrust" (44, a). It is doubtful whether he considered the structure 

 of the Cretaceous rocks of the eastern belt as defined above to be 

 the factor of prime importance in the explanation of the structure 

 of the region, that it undoubtedly is. For example, in describing the 

 structures beneath the Lewis thrust, he states (45, g) : "The structure 

 of the Cretaceous beneath the Lewis thrust was not connectedly 

 observed. . . . Out of perhaps twenty reliable observations of dip, 

 distributed over the entire area of Cretaceous sub-terrane, nine- 

 tenths are to the southwest and are from a degree to 25 degrees. 

 In the field the monoclinal southwestern dip was taken to he a simple 

 structure." (The italics are mine). He recognizes, however, that this 

 supposed monocline is complicated either by eastward dips in part of 

 the area or by other overthrusts. 



The Cretaceous area adjacent to the International Boundary 

 studied by Willis has a more simple structure than has the Cretaceous 

 either to the north or to the south. The simple structures at the 

 boundary are not characteristic of the disturbed belt of the Cretaceous 

 either in Montana or Alaska. 



The three fundamental assumptions on which Willis based his 

 explanation of the structure of what is here termed the Central area, 

 as stated by himself (45, a) are: "(a) The thrust surface coincides 



