Folds are always the result of compression forces. The two most common folds are anticlines and synclines. Sometimes it is not possible to see these structures in their entirety yet it is possible to interpret what type of structure is present by looking at the outcrop pattern on the land's surface. Simple folds will yield a striped outcrop pattern on the land's surface as these folds erode away. In an anticline the older rocks are found at the center of the structure and become progressively younger as you move outward. In a syncline the younger rocks are found at the center of the structure and become progressively older as you move outward.

A plunging fold is one in which the fold axis is tilted from the horizontal. In this case the outcrop pattern becomes more complex. 

Sometimes rocks will become folded in all directions to form bowl-shaped features. These are called domes (when the bowl is upside-down) and basins (when the bowl is right-side-up).  In the case of domes and basins the surface pattern of geologic structures will be a circular bulls-eye pattern. In a geologic dome, the oldest rocks will be found in the center whereas in a basin the youngest rocks will be found in the center.