RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 360 
Burning of this coal would produce power enough to move the 
block 2.2 feet, assuming the usual engine efficiency. It has 
actually moved an average of at least six miles. 
It is especially interesting to note that the force required to 
shear the block loose over the whole area is only about one-tenth 
of that required to produce motion against the resistance of friction. 
Since both forces are proportional to area and only one—that of 
motion against friction—proportional to thickness, we find that 
for a block of any area and of a thickness of 287 feet, according 
to the conditions assumed, the shearing force is just equaled by 
the force to overcome friction, and as thickness is greater than 
this amount the latter force is greater in proportion. It is evident, 
then, that in the case of most overthrust faults the motion of the 
rock involved against the resistance of friction is more impressive 
than the production of the break which separated it. 
