CONTORTIONS OF THE STRATA 



From this succession of episodes it is seen that a break of this 

 kind between two series of deposits involves a double oscillation 

 of subsidence followed by elevation a large depression followed 

 by a large elevation, a smaller subsidence followed by elevation. 

 The time interval which must have been represented by these re- 

 peated operations is so vast as at first to stagger the mind in con- 

 templating it. When, as in this instance, the dips of the lower 

 series of beds differ from those of the upper, we have to do with 

 an angular unconformity. It may be, however, that the lower 

 series was not so far depressed as to enter the zone of flow, and 

 that its beds meet those of the upper series with apparent con- 

 formity. Such an unconformity is often extremely difficult to 

 recognize, and it is described as a deceptive or erosional uncon- 

 formity. 



With a deceptive unconformity the clew to its real nature is 

 usually some fact which indicates that the lower series of sedi- 

 ments had been raised above the _ -.-.=-_^ = ~_ ^- 



level of the sea before the upper A' : '~-^j^T' ''""'"' ~:E]'^: ^.'~_m 

 series was deposited upon it. 

 This may be apparent either in 

 the irregularity of the surface on 

 which the two series are joined, 

 in some evidence of the action 

 of waves such as would be fur- 

 nished by a basal conglomerate 

 in the upper series, or some in- 

 dication of different resistance of 

 different rocks of the lower series 

 to attacks of the atmosphere 

 upon them (Figs. 33 and 35 a-c). 



In most cases, at least, the 

 lowest member of the upper 



series will be a different type of 



i /. . i 



rock from the uppermost mem- 



ber of the lower series, hence the 



frequent occurrence of the dis- Fio.35. Types of deceptive or erosional 



cordant cross bedding in sand- unconformities. 



stone should not deceive even the novice into the assumption 



of an unconformity. 



illlll 



-1 , ' i ' r 



. B///owy ^Surface. 



