54 New York State Museum 



Faults are fracture planes in the earth's crust along 

 which there has been displacement of strata (figure 9). 

 The surface along which movement takes place is the 

 fault-plane or fault-surf-ace. When there are a series of 

 faults together, each showing a small amount of slipping 

 in the same direction the faults are known as step- faults 

 and the region covered by the entire series as the fault 

 cone. The surface along which movement takes place is 

 polished, striated and grooved. The term slickensides is 

 used for such surfaces. The intersection of the fault- 

 plane with a horizontal plane or surface gives the fault- 

 line and its direction is known as the trend (sometimes 

 strike). The dip of the fault-plane, that is its departure 

 from the vertical, is known as the hade. The term dip is 

 restricted to inclined strata. Where there is an inclined 

 fault-plane or surface the side projecting below is called 

 the foot wall, the overhanging side the hanging wall. The 

 vertical distance between the same strata on each side of 

 the fault-plane is the amount of vertical displacement or 

 throw. The amount of displacement in a horizontal plane 

 is known as the heave. There is no heave when the fault- 

 plane is vertical. In faulting there may be movement on 

 both sides of the fault-plane or one side may be stationary 

 and the other side may move up or down. There are two 

 main kinds of faults, normal and reverse. In a normal 

 fault the hanging wall or overhanging side slips down 

 with reference to an assumed stationary foot wall. This 

 is also known as a gravity fault. In a reverse fault the 

 hanging wall moves up. Reverse faults are found most 

 frequently in regions where much crushing and folding 

 has taken place. When the fault-plane is gently inclined 

 to nearly horizontal such faults become thrust faults, 

 which are sometimes of great magnitude involving regions 



