CoachA fold is a bend of once-planar layers. An anticline is a fold whose layers are convex upward (an upfold); a syncline is a fold whose layers are concave upward (a downfold). The two sides of a fold are its limbs; its fold axis is the line along which the layers bend most, and the axis plunges when it is not horizontal. A line such as a fold axis is located by its trend, the azimuth of its horizontal projection taken in the downward direction, and its plunge, the angle it makes with the horizontal (Chapter 6). Fold geometry in three dimensions is the subject of Chapter 17.
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Example 4.18 (The history of the Varden Hills)
On the map of the Varden Hills: the schist is the oldest unit; the granite cuts it, so it is younger; the sandstone rests on both, cutting off the granite’s outline: an unconformity, after a long erosion. The sandstone, limestone, shale and chalk follow one another without a break and are folded together into an anticline (sandstone and basement in the core, near ) and a syncline (chalk in the core, east of the map’s centre): the folding came after the chalk was laid down. The fault offsets the folded contacts: it is younger than the folding. Finally the river has cut down and spread its alluvium, which no fault breaks.