INTRAFORMATIONAL CORRUGATED ROCKS 605 



Such contorted zones were quite certainly produced before the 

 overl3dng clays were deposited, as proved by the notable variation 

 in thickness of the contorted zone, its very irregular upper surface, 

 and the manner in which the perfectly undisturbed overlying clays 

 were laid down on the irregular surface. 



W. A. Johnston^ has described and figured an interesting case 

 of notably crumpled sand forming a zone of variable thickness under 

 till near Fort Frances, Ontario. He says that the crumpling of the 

 sand was due to the over-riding action of glacial ice. 



DIPFERENTIAL WEIGHTING 



Kindle,^ in his discussion of deformation of unconsoHdated beds 

 on the Avon River, Nova Scotia, describes "a section of finely 

 laminated horizontal silts, which, for a thickness of one foot or 

 more near the middle, have been distorted into a highly convoluted 

 zone." He advances the hypothesis of differential weighting to 

 account for the phenomenon and says : 



If a heavy load of sand were deposited over a portion of an area in which 

 very soft beds were interpolated between more coherent strata, the more mobile 

 wotild be likely to squeeze outward away from the sand pressure toward an 

 unsupported edge, if one were developed by stream or wave cutting. This 

 might occur without disturbing firmer beds above and below through the more 

 yielding character of the soft beds. 



According to Kindle, current scour would remove the heavier and 

 coarser beds, after which horizontal layers would be deposited over 

 the disturbed beds. He describes experiments in which clay beds in 

 glass tanks were notably deformed by differential weighting with 

 shot. 



Some reasons for thinking that the foregoing explanation is not 

 applicable to the Avon River occurrence, and quite certainly not to 

 intraformational contorted clays in general as typified by the Con- 

 necticut Valley occurrences above described, are as foUows: (i) 

 Quite generally, in nature, there is no evidence of anything like 

 notable current scour, but rather there has been continual deposition 

 of the clays; (2) in the experiments the surfaces of the deformed 



' W. A. Johnston, Can. Geol. Surv., Mem. 82 (1915), p. 43 and PL 8. 

 " E. M. Kindle, Geol. Soc. Amer. Bull., Vol. XXVIII (1917), PP- 323-32. 



