2IO 



THE BOOK OF NATURE STUDY 



older rock was folded and denuded before the newer was laid 

 down on its denuded edges (Fig. 82). The deposition of the 

 older bed must have occurred under water in an area of rest. 

 It must have been folded by earth movement to give it its dip, 

 and it must have been lifted up within the area of denudation, 

 that is into shallow water or to form a land surface. After de- 

 nudation it must have been again lowered beneath the water into 

 a new area of deposition. Much time must therefore have elapsed 

 between the two sets of strata, possibly represented by deposits 

 elsewhere. There is a gap in the succession, and, what is of more 

 importance from our present point of view, earth-movement must 

 have occurred in the interval. 



Now, it is such earth-movement which lifts sea beds to form 

 continental masses, and elevates plateaux and mountain chains. 

 Consequently the occurrence of unconformities between strata 



FlG. 81. Unconformable junction of beds b with beds a. 



enables us to date the origin of the features which result from 

 earth-movement, while it furnishes new information as to the 

 boundaries of sea and land after and during the movement. From 

 such evidence we know that there have been four great and 

 several minor periods of earth-movement in Britain. Moreover, 

 we know that each period of earth-movement was a prolonged 

 cycle, taking sometimes several geological Periods for its com- 

 pletion. 



Each earth-movement contributed new landscape features to 

 the country ; some of them were transient, but many have 

 survived to this day and still form conspicuous and important 

 features in the geography of the country. The movements must 

 necessarily have reacted on the progression of animal and plant 

 life ; they must have stimulated or retarded evolution ; and must 

 have had much effect on the climate and denudation, and on the 

 drainage and relief of the country. The earliest earth-movement 

 of which we have knowledge preceded the Cambrian Period ; the 



