52 



EARTH FEATURES AND THEIR MEANING 



Here it is evident that the sediments which compose the lower 

 series of beds have been folded in the zone of flow, though the 

 upper series has evidently escaped this vicissitude. Furthermore, 

 the surface which delimits the lower series from the upper is some- 

 what irregular and shows a hard layer standing in relief, as it 

 would if it had opposed greater resistance to the attacks of the 

 atmosphere upon it. 



In reality, an unconformity between formations must be in- 

 terpreted to mean that the lower series is not only older than the 

 upper, as shown by the order of superposition, but that the time 

 of its deposition was separated from that of the upper by a hiatus 

 in which important changes took place in the lower series. The 

 stages or episodes in the history of the beds represented in 

 Fig. 33 may be read as follows (see Fig. 34 a-e) : 



(a) Deposition 



of the lower series 

 during a transgres- 

 sion of the sea. 



(6) Continued 

 subsidence and 

 burial of the lower 

 series beneath 

 overlying sedi- 

 ments, and flexur- 

 ing in the zone of 

 flow. 



(c) Elevation of 

 the combined de- 

 posits to and far 

 above sea level and 



> 



v ~. . , ,. removal by erosion 



1 10. 34. Series of diagrams to illustrate in succession the - , , , '. -, 



episodes involved in the historical development of an ot Vast thicknesses 



angular unconformity. The vertical arrows indicate of the Upper sedi- 



the direction of movement of the land, and the horizontal me nts 

 arrows the direction of shore migration. 



(a) A new sub- 

 sidence of the truncated lower series and deposition of the upper 

 series across its eroded surface. 



(e) A new elevation of the double series to its present position 

 above sea level. 



