286 ELEMENTARY GEOLOGY 
other. In some cases this goes so far that the folds are 
inverted or overturned * (Fig. 168). Indeed, some rocks 
are so excessively folded that they are really crumpled, 
just as we might crumple a piece of paper (Fig. 169), 
This crumpling of rocks is particularly common among 
those which are 
metamorphosed, 
and is produced 
deep down in 
the earth. 
It seems 
strange to think 
of rocks as fold- 
ing like so much 
paper; but even 
brittle substances can be bent and folded if they are 
Section through a monoclinal fold. 
prevented from breaking by a great pressure on all 
sides, and if the change is slowly introduced, as seems 
usually to be the case in the folding of the earth’s 
crust. It is somewhat like the action of ice, which, 
though naturally brittle, may be easily bent if this is 
1Jt is not to be understood that rocks are commonly overtoppled; for 
it is probable that these overturned folds are always formed deep below 
the surface. Even if they were produced at the surface, it is to be remem- 
bered that mountain folds take shape with extreme slowness, and that, 
as they grow upward, they are also being cut down by denudation; so 
that they do not reach the elevation that they would attain if nothing 
interfered. Thus an overturned fold may be formed without any actual 
overtoppling. 
