INTRAFORMATIONAL CORRUGATED ROCKS 597 



Even in this case some shearing action or differential slipping was a 

 factor in the process. 



Prouty^ has described the crumpling of thin beds of marble 

 between thick beds which latter yielded by fracturing and faulting. 



The principle of differential squeezing appears to be not uncom- 

 mon in various regions of folded strata. 



DIFFERENTIAL MOVEMENT UNDER THE ACTION OP GRAVITY 



Fine examples, believed by the writer to belong in this category 

 of intraformational contorted strata, are to be found in the post- 

 glacial clays of various regions. The following observations made 

 by the writer upon the clays in and near Northampton, Massachu- 

 setts, give a fair idea of the nature of the foldings and their origin. 

 In the bank of the Connecticut River 2 miles east of Northampton, 

 12 to 14 feet of nearly horizontal, thin-bedded, perfectly stratified 

 clays are overlain by 10 to 15 feet of stratified sands. The clay 

 contains two contorted zones — a lower one i to 2 feet thick, and an 

 upper one 4 to 8 inches thick — separated by 8 or 9 feet of the 

 ordinary non-contorted clay. These corrugated zones are clearly 

 traceable for several hundred feet. Immediately above each con- 

 torted zone, the non-contorted layers are in many places somewhat 

 wavy or slightly folded. Different portions of the same contorted 

 zone show different degrees of folding, the clay beds in some cases 

 being only moderately folded, while in others they are intensely 

 twisted, pulled apart, and even overturned. Some of the straight 

 beds contain notable amounts of very fine sand, but the contorted 

 beds consist of distinctly less sandy clay. The corrugations almost 

 invariably have strikes parallel not only to each other but also to 

 the notable dip (several degrees) of the clay beds in general. The 

 under surface of each contorted zone is usually very straight, while 

 the upper surface is commonly somewhat irregular (Fig. 5). 



In the South Street clay pit of Northampton the writer has ob- 

 served a very fine highly contorted zone of clay sharply inter- 

 calated between beds of clay whose stratification surfaces are almost 

 straight. The straight beds consist of alternating, very fine-grained, 



^ W. F. Prouty, Geol. Surv. Ala., Bull. 18 (1916), p. 170. 



