MECHANICS OF FORMA TION OF ARCUA TE MOUNTAINS i8i 



Attenuation of under limb of anticline after under turning. — There 

 are still other consequences of the rapid loss of competence of an 

 anticHne after underturning has begun. The active force of 

 compression, now no longer deflected upward into the first anticline 

 but transmitted along its original direction, tends to reduce the 

 volume of the included arches of inferior strata. The resistance 

 which they offer to this reduction of volume tends to extend 

 (stretch) the under limb (Fig. 22). It is a very general observation 



that thinning of the under limb is 

 characteristic of so-called "over- 

 turned" anticlines, and emphasis 

 may here be laid upon the point 

 that, were anticUnes really over- 

 turned, as has been so generally 

 supposed, it is the upper limb which 

 should be attenuated by the process, 

 and not the lower (Fig. 23).^ 



Though closed anticlines with at- 

 tenuated upper limbs have, so far as 

 known, never been observed in folded 

 rock formations, they are, on the 

 other hand, the characteristic type of close recumbent folds in the 

 ice of glacier snouts, where the action of 

 gravity and the resistance of friction in 

 the lower layers force the upper layers 

 to override the lower in true over- 

 turning movements (Fig. 24, p. 182).^ 

 Not only may the lower limb of 

 underturned anticlines be attenuated 

 by stretching due to the resistance of 

 the inclosed inferior strata, but the upper Hmb may in this stage be 

 bulged upward as a result of the same system of forces. 



' See also ante, p. 82, and Fig. 7. The sinking of the crown in an anticline tends to 

 develop a tension within the upper limb, and it is perhaps conceivable that this tendency 

 might in some stage either equal or overbalance that which normally causes attenuation 

 of the lower limb, yet so far as known no example is furnished by folded strata. 



" See among other views which show this effect: Med. om. Gronland, XLVI (191 2), 

 Fig. 24; The Alpine Journal, London, XXI, 187; Chamberlin and Salisbury, Geology 

 I, 280, Fig. 268. 



Fig. 22. — Stretched under limb 

 of a recumbent anticline. Drawn 

 from a photograph of the Dent de 

 Morcles as seen from Les Marti- 

 nets, western Switzerland. 



Fig. 23. — Attenuated upper 

 limb of an anticUne, a neces- 

 sary consequence of overturning. 



