Folds form in layered rocks when compressional forces squeeze strata like a rug can be pushed to form "ripples" in the rug. Anticlines are folds whose flanks dip away from the hinge line (the bend) of the fold. Synclines are folds whose flanks dip toward the hinge line of the fold. Anticlines and synclines share flanks, as folds alternate anticline, syncline, anticline, syncline, and so on. View the following animation to appreciate the process of folding through compression (Observe the shortening that happens, the squeezing into a more compact volume):
In the set of folds shown in these illustrations, there is an anticline closest to the viewer, a syncline in the middle, and another anticline on the other side. Erosion will eat down into such folded strata as a landscape forms after mountain building and folding are over. The planar surface in the following illustration represents such a landscape surface, which we will treat as planar to make it simple. The folds shown are nonplunging. This means that their hinge lines, or axes, are horizontal or nearly so.
An outcrop pattern for nonplunging folds will consist of simple straight lines. If erosion were to bring a landscape surface down to the level of the plane shown in the illustration above, a straight line outcrop would be present, with alternation of the same strata:
Folds which have been tilted or bent (essentially, folding of folds), which happens during and sometimes after the original episode of folding, are said to plunge. Their hinge lines, or axes, dip into the ground. You can simulate this by folding a sheet of paper as in the animation above, then tilting the set of folds one way or the other so they plunge. Take a look at a set of plunging folds:
An outcrop pattern developed on such a geologic structure, if erosion eats down to the plane shown, will be conspicuously "zig-zagging":
Nonplunging, or only gently plunging folds are very common. Plunging folds are not rare, but are restricted to areas where deformation has been more complex, with several episodes of folding along different compression directions.