INTRAFORMATIONAL CORRUGATED ROCKS 



599 



weight of overlying materials being far more plausible; (2) there 

 are no notable irregularities or depressions at the tops of the corru- 

 gated zones such as must have developed in the case of crumpling 

 of surface layers with these depressions first filled by the succeeding 

 deposits; (3) the moderate disturbance of the beds immediately 

 overlying the corrugated zones, while those just underneath are 

 distinctly straighter, strongly point to differential movements after 

 the overlying beds were laid down; and (4) the only slightly dis- 

 turbed thin bed of fine sandy clay in the midst of the contorted 

 South Street clays, as well as the relatively straight beds of similar 

 material just above and below the contortions, are best accounted 



Fig. 6. — Contorted zone in South Street clay pit of Northampton, Massachusetts 



for on the basis of differential movements, the thin, clay-rich, 

 very plastic beds having yielded by crumpling, while the much less 

 plastic sand-rich beds did not crumple. It is out of the question 

 to look upon the upper surfaces of the corrugated zones as erosion 

 surfaces because these perfectly stratified, thin-bedded, postglacial 

 clays ever3rwhere plainly show that they were deposited without a 

 break in very quiet water. 



The hypothesis of differential movement does not necessarily 

 preclude the possibility of subaqueous slumping for it is plausible 

 to think of differential movements within masses of the clays which 

 may have shifted more or less down the gently sloping delta fronts 

 in the postglacial lake of the Connecticut Valley, The action of 

 gravity alone, or of gravity aided by an occasional earthquake, 

 may have caused the movements. It is more likely, however, that 



