94 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Though inclined to favour the two-episode hypothesis Willis was careful 

 to leave the question open. The problem is complicated by the apparent neces- 

 sity of believing that, at a late Paleozoic (post-Mississippian) stage the whole 

 Purcell-Rocky Mountain province was broadly upwarped, forming a geanticlinal 

 area, from which the sea was excluded during the Triassic and Jurassic periods. 

 There thus seems to be a possibility that tbe structural condition for the Lewis 

 thrust — an antecedent flexure near the site of the present frontal escarpment 

 of the Rockies — was established long before the Laramie or the Cretaceous 

 period. To the present writer the view that the post-Cretaceous movements all 

 belonged to a single orogenic episode has the merit of simplicity and does not 

 seem to be contravened by any known fact. Moreover, one observation in the' 

 field seems to indicate directly that great thrusts were developed in the Clarke 

 range either during the main post-Laramie folding, or at any rate, before that 

 folding was completed. 



This partial evidence is illustrated in the structure section of Figure 6 

 and map sheet No. 1. It will be observed that a heavy wedge of Siyeh lime- 

 stone has been pushed eastward along a nearly horizontal thrust-plane, which 

 has truncated a thick mass of the Altyn beds. The wedge has penetrated the 

 overlying Appekunny beds, crumpling, mashing, and forcing them aside like 

 an enormous plowshare. An inspection of the whole section seems to warrant 

 the conclusion that this thrust which forced the younger Siyeh limestone into 

 contact with the older Altyn and lower Appekunny beds, must have antedated 

 the development of the Cameron Falls anticline in its present structural form. 

 It is hard to believe that the limestone wedge was driven downwards to an 

 actually lower level. A more reasonable view is that the Altyn- Appekunny 

 beds on the eastern side of the present fold were formerly faulted or bent 

 upwards so as to feel the thrust of the still flat-lying Siyeh limestone on the 

 west. After the wedge was intruded and by the continuance of the same com- 

 pressive stress, the whole series was flexed so as to show the existing anticlinal 

 structure. On the probable supposition that this thrust and the Lewis thrust 

 were contemporaneous, it follows that the latter was also contemporaneous 

 with an early stage in the main folding of the Clarke range. 



At other points in the Clarke range as well as in the Galton-Macdonald 

 mountain group the geosynclinal rocks underwent a powerful compression 

 before they were folded at all. At these localities the dip of the bedding is 

 20° or more, while marked slaty cleavage has been developed with its plane 

 characteristically perpendicular to the bedding-plane. It seems most probable 

 that this cleavage was the product of a compressive force which was directed 

 along the bedding-planes and had developed the cleavage before significant 

 upturning occurred. This phase of tangential compression may be that in 

 which the great Siyeh wedge was thrust into the Appekunny metargillites and 

 in which the much greater Lewis thrust was formed. Most of the folding may 

 have been produced as a slightly later expression of the same but somewhat 

 intensified, tangential force. On this view both thrust and fold belong to one 

 orogen:c episode. 



