272 G. R. MANSFIELD STRUCTURE OE ROCKY MOUNTAINS 



OVERTHRUSTING VERSUS IJnDERTHRUSTING 



Current views regarding overthrusts have in recent years been chal- 

 lenged. Thus Hobbs^^ some years ago and more recently Lawson^^ have 

 argued that rocks are underturned and underthrust rather than over- 

 turned and overthrust. Mathematical studies of such problems as over- 

 thrusting, however logical, are not always convincing, since the results 

 obtained must depend on the validity, comprehensiveness, and accuracy 

 of the assumptions on which the calculations are based. In the Scottish 

 Highlands the imbricated structure appears to bear out Lawson's view 

 that a great wedge of the superficial crust will break up under powerful 

 tangential pressure into smaller blocks that will stand at high angles with 

 regard to the original thrust plane. It shows further, however, that these 

 smaller blocks, when in this position, develop resistances that accumulate 

 in such manner that the blocks may act as a single unit, which, under 

 sufficient tangential pressure, is moved forward with reference to under- 

 lying structures. 



In most so-called overthrusts the upper block consists of older rocks, 

 more massive and rigid than those that underlie them. It is easier to 

 think of these older rocks, under tangential pressures such as produce 

 these great dislocations, as moving upward and outward toward the sur- 

 face, where resistance is less, than to suppose that weak, incompetent 

 formations are forced inward and downward into zones of increasing 

 resistance. 



With the supposition of underthrusting it is presumed that the greatest 

 effects would be felt near the zone of application of the tangential forces 

 or, in the case of the northern Eockies, along the eastern border of the 

 mountains. If it be presumed further that the zone of application of 

 effective forces might migrate inward — that is, westward — it should be 

 possible to discover relatively younger dislocations in that direction. So 

 far as the facts are known, there seems to be no greater intensity of 

 mountain-building along the eastern border of the Rockies than farther 

 west. Moreover, the Heart Mountain overthrust and possibly the Lewis 

 overthrust, which mark the outer border of the known great overthrusts, 

 are of later date than the Bannock overthrust, and this in turn is probably 

 somewhat later than the Philipsburg faults. Thus it seems that the zone 

 of thrusting has moved progressively eastward rather than westward. 



22 W. H. Hobbs : Mechanics of formation of arcuate mountains. Jour. Geol., vol. 22, 

 1914, p. 206. 



23 A. C. Lawson : Isostatic compensation considered as a cause of thrusting. Bull. 

 Geol. Soc. America, vol. 33, 1922. pp. 337-352. 



