254 



PHYSICAL GEOLOGY 



at the surface, the smaller the dip the wider being the outcrop. 

 For example, a bed one foot thick at an angle of i° on perfectly level 

 ground has an outcrop 40 feet wide; with a dip of 5 , the outcrop is 

 about 1 1 feet wide ; with one of 30 , it is only 2 feet, and when the 



stratum is on edge 

 (90 dip), the thick- 

 ness of the bed deter- 

 mines the breadth of 

 the outcrop (Fig. 



' -v 





4 f^Si 









240). 



Folds 



Fig. 241. — A sharp anticlinal fold. (After E. A. N. 

 Arber, The Coast Scenery of North Devon.) 



When inclined 

 strata are cut by 

 streams or eroded by 

 waves, they are often 

 seen to be arched or 

 in anticlines (Fig. 

 241), or in troughs or 

 synclines (Fig. 242). 

 These anticlines and synclines vary in size from a few feet to scores 

 of miles across, and from almost flat arches to such steep ones as that 

 shown in Fig. 348, p. 360. In an anticline the strata dip from both 

 sides of the crest line called the axis of the fold, and in the syncline 

 towards the axis of 

 the trough. When 

 folds are closely com- 

 pressed, so that the 

 flanks or limbs are ap- 

 proximately parallel, 

 they are called iso- 

 clines. Folds are not 

 always symmetrical, 

 but, on the contrary, 

 are often unsymmet- 

 rical (Fig. 238, p. 

 252). When they are 

 so inclined that one 



limb or flank becomes FlG 242 . _ A synclinal fold, North Devon, England. (E. 

 doubled under the A. N. Arber, The Coast Scenery of North Devon.) 



