STRUCTURAL FEATURES 



275 



-, 



Fig. 251. Isocline. 

 U. S. Geol. Surv.) 



(Vanllisr. 



Many sedimentary rocks and many lava flows have lost their 

 original position through crustal movements, so that beds which were 

 nine horizontal now dip; that is, they depart from horizontality. 

 The beds of a given region may all 

 dip in one direction, or the dip may 

 change from point to point. They 

 may be folded, and the folds may be 

 open (Fig. 249) or closed (Fig. 250). 

 The beds of sedimentary rock may 

 even be on edge (Fig. 251), having a 

 dip of 90. These diverse positions 

 in which strata are found are the 

 result of disturbance subsequent to their deposition. 



Modifications of the original attitude result from earth move- 

 ments, and the measurement of these modifications is an important 



part of field study. 

 The position of beds 

 is recorded in terms of 

 dip and strike. The 

 dip is the inclination 

 of beds referred to a 

 horizontal plane (Fig. 

 "''-''"" 252) and is usually 



measured by a clino- 

 meter. In measuring 

 the dip, the maximum angle of slope is always taken, and its direc- 

 tion as well as its amount is recorded. Thus dip 40, S. 20 W., 

 gives the full record of the position of the bed of rock under con- 

 sideration. The strike 

 is the direction of the 

 horizontal edge of dip- 

 ping beds, or more gen- 

 erally, the direction of a 

 horizontal line along the 

 outcropping edge of a 

 dipping bed, as illus- 

 trated in Fig. 252. Since 

 the strike is always at 



Fig. 252. Diagram illustrating dip and strike. 



Fig. 253. Recumbent anticline. 

 U. S. Geol. Surv.) 



(Van Hise, 



right angles to dip, strike need not be recorded if the direction of the 

 dip is. Thus dip 40, S. 20 W. is the same as dip 40, strike N. 70 \Y. 



