THE STRUCTURE OF THE EARTH 



271 



in Fig. 272 we learn (1) that there was a long period of quiet, during 

 which the lower series of beds were deposited continuously on the 

 ocean bottom. This was followed (2) by a period of folding or tilting 

 and of elevation, so that these beds were raised above the level 

 of the sea. (3) The strata were then subjected to erosion for such 

 a long period that the upturned edges were worn to a peneplain. 



Fig. 272. — An unconformity in which the lower beds are inclined, while 

 the upper are horizontal. Wyoming. (U. S. Geol. Surv.) 



How long this erosion interval lasted cannot be told from any one 

 geological section. (4) The land was later depressed and became 

 sea bottom, so that (5) sediment was laid down on the old land sur- 

 face. (6) Reelevation again converted the sea bottom into land, 

 and streams are now at work carrying the rock back to the sea. 



Overlap is a term used in describing an unconformity in which the 

 younger strata cover a larger area than the older ones and conse- 



F D 





B 



Fig. 273. — Diagram showing the overlapping of the^trata AEDC due to the encroach- 

 ment of the sea upon the ancient land surface AC. (Modified after Grabau.) 

 CLELAND GEOL. — 1 8 



