DEFORMATION OF ROCKS 315 
Margerie and Heim as follows: An ordinary fold is one in which 
the strata diverge from the crest of the anticline and the trough 
of the syncline (Figs. 2-4). Ordinary folds may be described as 
Fic. 2.—Simple inclined fold. 
gentle, open, or close. In close folds, according to Willis, the 
process has gone so far that the strata are perceptibly changed 
in thickness in different parts of the fold. An isoclinal fold 
——— i 
SS 
Fic. 3.—Simple overturned fold. 
is one in which the strata are parallel, or nearly so (Fig.5). A 
fan fold is one in which the strata converge downward from the 
crest of the anticline (Fig. 9), or upward from the trough of 
the syncline. In this case the strata at the limbs of the fold 
are always greatly thinned, and in some instances the central 
strata are absent, the material having flowed up and down, form- 
