[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 123 



Mesozoic upper beds. A feature of the general structural relations 

 that seems anomalous is the fact that the most severe deformation is 

 now exposed in the eastern zones of the disturbed area, or those 

 farthest from the old land to the west from which the thrust presum- 

 ably came. Experimental work indicates that horizontal stresses 

 are not propagated far forward into a mass of strata (7), yet the field 

 facts indicate that if the stress did come from the old land, its effects 

 were felt many miles to the eastward. It may be that the especially 

 competent Precambrian and Palaeozoic limestones, thickened and 

 strengthened by the earlier events of compression, did transmit the 

 forces far eastward, or it may be that an access of stress, acting from 

 below diagonally upward in the neighbourhood of the MacDonald 

 range, severely compressed the zones to the east. Stresses were, at 

 any rate, acting in an eastward direction throughout what is now the 

 Rocky mountains. 



The structures exhibited by the central and eastern structural 

 areas of this region when considered as a whole, bear a remarkable 

 similarity to the structures of certain areas of the greatly overthrust 

 portions of the northwest highlands of Scotland. In the monumental 

 memoir of the Geological Survey of Great Britain on the geological 

 structure of the northwest highlands of Scotland (29) numerous 

 figures are given which illustrate the occurrence of flat overthrusts of 

 large displacement underneath which are very steep reverse faults 

 of small displacement. The scale is, of course, less in the Scottish 

 examples than in the Rocky mountains, and the intensity of over- 

 thrusting and accompanying reverse faulting is much greater, as there 

 are several major overthrusts overlapping in Scotland where one only 

 is known in Alberta. In spite of these differences, however, the 

 essential features of the two regions are similar. 



Although the structures of the Rocky mountains are on a larger 

 scale, their relatively greater simplicity invites an explanation of the 

 mechanism of their formation along the lines suggested by the experi- 

 ments of Cadell (7), whose summarized conclusions are given here 

 (20, b). 



1. Horizontal pressure applied at one point is not propagated far forward into 

 a mass of strata. 



2. The compressed mass tends to find relief along a series of gently-inclined 

 thrust-planes, which dip towards the side from which pressure is exerted. 



3. After a certain amount of heaping-up along a series of minor thrust-planes, 

 the heaped-up mass tends to rise and ride forward bodily along major thrust-planes. 



4. Thrust-planes and reversed faults are not necessarily developed from split 

 overfolds, but often originate at once on application of horizontal pressure. 



