STRATIFIED OR SEDIMENTARY ROCKS. 



/J/2,3'44.3 i.^ 



Fig. 151. 



example, if the strata are lifted up in the form of a cone or a dome, 

 the strike will be circular ; if the strata be folded and then tilted in a 



direction at right an- 

 ,.-:^;:"S\ -, .- -';-'-'-::::;; :: n>. /,<;"' :;:: Cv: x'*/--;"-.^ gl es to the folding 



force, the strike will 

 be sinuous. Again, 

 if the surface of the 

 ground is level, the 

 outcrop will be the 

 strike, and will be straight or sinuous, according as the strike is one or 

 the other. But the outcrop is usually far more irregular than the 

 strike, as it is affected also by irregularities of surface produced by 

 erosion ; so that in a broken country the outcrop of folded strata is ex- 

 tremely complex. 



Anticline and Syncline. — Strata are usually more or less folded, and 

 therefore form alternate saddles and troughs. The saddles are anticlines, 

 the troughs synclines. The line aloug the top of the saddle is an anti- 

 clinal axis, the line along the bottom of the trough a synclinal axis. 

 If it were not for erosion, the anticlines would be ridges and the syn- 

 clines valleys. If erosion cuts down to nearly a plane, as in Fig. 152, 

 then an anticline is 

 known by the strata 

 being repeated on 

 each side of an axis 

 and dipping away 

 from one another ; a 

 syncline by the strata 

 also repeated on each side of an axis but dipping toward one another. 

 In the one case the oldest and loivest strata are on the axis and they 

 become higher and newer as we go either way ; in the other the upper- 

 most and newest strata are along the axis, and they become lower and 

 older as we go either way. 



But erosion usually forms ridges and valleys. In this case some- 

 times the ridges are anticlines 

 and the valleys synclines, as 

 in Fig. 153, but sometimes the 

 reverse is true. It is very 

 common to have synclinal 

 ridges and anticlinal valleys 

 (Fig. 154). In this case the 

 original configuration is completely reversed by erosion. This will be 

 explained in another chapter (p. 268). 



Fig. 153 and the following figures (155 and 156) will illustrate some 

 of these points. Suppose we have strata gently folded, so as to make 



Fig. 152. 



Fig. 153.— Section of Undulating strata. 



