316 STUDIES FOR STUDENTS 
ing detached arch cores and detached trough cores. An ordinary, 
isoclinal, or fan fold may be upright, inclined, or overturned. 
In the formation of the simple fan-shaped anticline the 
Fic. 4.—Simple recumbent fold. 
rocks are extremely compressed on the limbs of the fold, while on 
the anticline the compression is not so severe. This is doubtless 
due to the partial escape from pressure of the material which 
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Fic. 5.—Simple isoclinal folds. 
rises into an arch, as compared with the deeper-seated mate- 
rial in the limbs of the folds, which constitutes a part of the 
continuous crust of the earth in which the major thrust must 
have been transmitted. Another factor is the relative strength 
