594 WILLIAM J. MILLER 



ments take place between the relatively more competent beds with 

 not uncommon development of corrugations of the nature of drag 

 folds between them. According to Leith: "The stronger beds tend 

 to assume the 'parallel' type of folds in which the principal adjust- 

 ment is between the beds rather than within them. This readjust- 

 ment or slipping is concentrated in the intervening weaker layers. 

 The folds of the weaker layers are really ' drag folds ' due to differ- 

 ential movement between the controlling harder layers."^ 



Excellent examples of intraformational corrugated strata 

 believed to have resulted from differential movement accompany- 

 ing regional folding were observed by the writer some years ago 

 at Baldhead Cliff near Ogunquit, Maine. The perfectly stratified 

 thin-bedded rocks are there interbedded quartzite and phyllite. 

 That the region has been subjected to severe lateral compression 

 is evidenced by the fact that the strata stand in nearly vertical 

 position. Figure 4 is a ground plan sketch showing the detailed 

 structure of one of these corrugated zones about 9 feet thick, 

 although the quartzite layers are really less conspicuous than indi- 

 cated in the diagram. The corrugated zone, consisting very largely 

 of phyUite, is not sharply delimited from the adjacent straight layers 

 of predominant quartzite on either side. In most cases slight 

 faulting has taken place along the axes of the sharp folds, but, as a 

 rule, individual sharp folds or faults rarely extend all the way across 

 the contorted zone. The folds are uniformly overturned toward the 

 east. Fracture cleavage is well exhibited in the phyllite layers a 

 few inches thick which lie within the quartzite on either side of the 

 contorted zone. The cleavage cracks are uniformly inchned toward 

 the east, as are the folds. "When a slate or shale is folded between 

 two competent layers, such as quartzite, the cleavage produced in 

 the slate affords clear evidence of slipping or shearing between the 

 quartzite beds."^ 



All the evidence points to the tectonic genesis of the above- 

 described intraformational corrugated strata. The corrugations 

 must have developed after all the strata were deposited because 

 the folded zone grades into the non-folded strata on either side. 



^ C. K. Leith, Structural Geology (1913), p. 114. 

 ^ Ibid., p. 119. 



