Folds

 Folds =  bends in a layer of rock due to ductile (plastic) strain.


Anticlines = folds pointing upwards.
Synclines = folds pointing downwards.
Limbs = sides or flanks of folds.
Hinge line = where the limbs of the fold meet. It is also the line of maximum curvature.

Axial Surface (or Axial Plane if it’s not curved) of a stack of folds passes through the hinge lines and most nearly divides the fold into two equal parts. This concept is best seen here.






Upright (Open) Fold: vertical/near vertical axial surface and limbs have similar dips (though in opposite directions!).

Inclined Fold: axial surface is neither vertical nor horizontal.

Overturned Fold: beds on one limb are upside down.

Recumbent Fold: axial surface is ~horizontal.

Isoclinal Fold: limbs are parallel.

Non-Plunging Fold: has a horizontal/near horizontal hinge line.
Anticline: older beds are in the core (center) of the fold and the beds dip away from the core.
Syncline: younger beds are in the core of the fold and the beds dip into the core.
Plunging Fold: has an inclined hinge line.
Plunging Anticline: beds point in the direction of the plunge; older beds are in the core (center) of the fold and the beds dip away from the core. Check out this classic example from Utah.
Plunging Syncline: beds point in the opposite direction of the plunge; younger beds are in the core of the fold and the beds dip into the core.
Plunging and non-plunging anticlines and synclines can be represented in 3D block diagrams to show how the folded strata behave beneath the surface.

Domes – a circular anticline (oldest beds in the core, beds dip away from the core).
Basins – a circular syncline (youngest beds in the core, beds dip into the core).

Domes and Basins can also be represented in 3D block diagrams to how how the folded strata behave beneath the surface.