MECHANICS OF FORMATION OF ARCUATE 

 MOUNTAINS 



WILLIAM H. HOBBS 



University of Michigan, Ann Arbor, Michigan 



PART II 



THE FOLDING PROCESS STUDIED IN THE PROFILE — GENERAL 

 CONSIDERATIONS 



Active and passive forces involved infolding. — So soon as we take 

 up the mechanics of rock folding, we again encounter the vital 

 question of the location relative to the fold of active and passive 

 forces in the process. Among the Swiss geologists it is universally- 

 held that the active force (Schub) which caused the folding and 

 slicing of the Alpine highland came from the southeast and was 

 directed toward the northwest. This view would appear to rest 

 upon the widely accepted notion that folds which are unsymmetrical 

 have been produced by an active force which operated from above 

 and behind the arch with the effect of pushing over the crown so 

 that in later stages it overhangs the base. This conception is 

 involved in the term "overturned fold" and its many variations.^ 

 If we may for the moment liken a fold to an overturned free wave 

 upon the surface of a body of water — a so-called "white cap" — the 

 active force which is generally assumed to produce the fold has the 

 same direction relatively as the wind. Like the wave, the fold 

 bends over toward the lee side because, as has been believed, the 

 active force operates above and directly upon the arch of the fold, 

 and not upon its base (Fig. 9, a and b). The effect of this system is 

 a couple — two parallel forces of which one is in this case a passive 

 force of resistance, which act in opposite directions and are sepa- 

 rated by a certain distance referred to as the arm of the couple. In 



' See Margerie et Heim, Les dislocations de Vecorce tcrrestre, essai de definition et de 

 nomenclature, Zurich, 1898, p. 54. Dr. E. A. Smith has, however, given the name 

 " underthrust folds," to what he evidently regards as exceptional cases of folding 

 (" Underthrust Folds and Faults," Am. Jour. Sci. (3), XLV (1893), 305-6). 



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