[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 113 



The displacement is known to be at least 15 miles; and in view 

 of the stratigraphie break of many thousands of feet, the low dip of the 

 fault surface, and its great lateral extent, Daly's suggestion may be 

 considered within the bounds of possibility. 



Structure Above the Thrust. — -In the vicinity of the North Kootenay 

 pass in Alberta there is no apparent disturbance of the beds above 

 the thrust surface that would indicate the presence of such an enormous 

 dislocation of the strata. The evenly layered sediments are here 

 traversed by numerous regular joints approximately at right angles 

 to the bedding. It is possible, however, that subsidiary slips and 

 fractures may be found on closer study. Near the 49th parallel 

 significant minor structures — minor only in comparison to the greater 

 break — have been described. Willis gives a clear description of these 

 structures above the break as observed by him, and his account is 

 quoted here (45, k). 



The detailed structure of the Algonkian mass above the Lewis overthrust is 

 sometimes chaotic when considered in the small, yet simple when observed in the 

 large. The chaotic structure is best exhibited in Chief mountain where the lower 

 massive member of the Altyn limestone is crushed. The fractures divide the masses 

 irregularly into blocks of all angular shapes varying from a few inches to 25 feet on 

 a side. The surfaces are slickened over wide areas, and where they preserve their 

 orientation in the clifïs the slickens demonstrate much relative horizontal dis- 

 placement of adjacent fragments. Certain fracture planes are in fact steep fault 

 surfaces along which displacement has occurred in the direction of the strike rather 

 than in that of the dip. Such faults are, however, without apparent system. In 

 other places, as north of Altyn, the cliflfs present mural faces traversed by remark- 

 ably regular lines of bedding which are crossed by nearly vertical joints. 



Viewed in the large, the structure of the Altjm limestone sometimes is that of 

 major and minor thrust faults. Yellow mountain, as seen from Chief Mountain 

 ridge, exhibits these relations very clearly. The basal major thrust lies at the foot 

 of the cliffs, somewhat obscured by talus, but sloping about 8 degrees in a curve 

 which on the left is less inclined and descends more rapidly to the right. Springing 

 from it are several minor thrusts, which dip more steeply and which upward pass 

 out either into the air or into an upper major thrust. The upper major thrust is 

 at the base of the argillites which dip gently and without appreciable disturbance 

 to the southwest. It simulates an unconformity. 



In Chief mountain a similar structure is more strikingly exhibited. The base 

 of massive Altyn limestone is traversed by minor thrusts which are often subparallel 

 to the bedding, so far as it can be made out. These thrusts dip 30 degrees and 

 occupy a zone about 1,000 feet thick above the Lewis major thrust. They are 

 limited above by an upper major thrust which is at the base of nearly horizontal 

 thin-bedded limestones, constituting the upper member of the Altyn formation. 



The thickness of strata within which major and minor thrusts are developed 

 is by no means constant. As stated, near Altyn the lowest beds of Altyn lime- 

 stone present mural regularity of structure, whereas in Yellow mountain probably 

 not more than 500 feet of strata are so repeated as to pile up 2,400 feet high. West 

 of Waterton lake, in the section seen by Dawson, the effects of minor thrusting are 



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