56 New York State Museum 



of enormous area. The fault-plane is then known as a 

 thrust plane. If faulting is more rapid than erosion an 

 initial cliff or fault scarp is formed. If erosion keeps 

 pace with faulting there will be no fault scarp except as 

 one may be exposed by later erosion along the fault line. 

 The motion along a fault-plane is not necessarily vertical 

 and according to the kind of movement horizontal, oblique 

 and rotary faults are developed. In a horizontal fault the 

 amount of movement along its strike is called the shove 

 or strike slip, and if the fault cuts across the bedding this 

 can be measured. More than one movement is probably 

 involved in most faultings. When the fault-plane is 

 parallel to the strike of the strata in tilted rocks the fault 

 is termed a strike fault. In a dip fault the fault-plane is 

 at right angles to the strike of the strata ; in an oblique 

 fault at an angle of about 45°. Through faulting and 

 subsequent erosion certain strata of rock in a region may 

 be eliminated; again there may be a duplication of beds 

 or the strata may not be continuous on opposite sides of 

 the fault etc. In thrusting there is a duplication of beds 

 and also an inversion of order, formations of older age 

 coming to rest upon younger beds. There are great over- 

 thrusts involving whole regions and where the fault-plane 

 is nearly horizontal they are difficult to recognize except 

 by the repetition of strata and relative ages of the beds 

 involved. Certain features accompany faultings, such as 

 slickensides, crumplings, fault breccias etc. 



Slaty cleavage is another structure produced in finer- 

 grained stratified rocks. When such rocks are sub- 

 jected to intense squeezing there is a flattening and 

 expansion of particles at right angles to the compres- 

 sion, resulting in the development of a secondary 

 parallel structure in the rock which has no relation to 



