overturned and recumbent

Folding may be so pronounced that strata are overturned beyond the vertical.

In an overturned fold, the beds dip in the same direction on both sides of the axial plane because one of those limbs being rotated through an angle of at least 90º. An extreme example of an overturned fold occurs when the axial plane is horizontal – this is called a recumbent fold.

(To visualize this concept, place an opened book flat on a table with the spine (hinge) uppermost. Close the book by squeezing the edges toward one another, and then flip the book onto one side. One half of the book (one limb) has rotated through more than 90º.)

[links: animations: folding; axial surface, plunge, fold; images: hand-specimen: overturned fold in limestone; formations: thrust and overturned fold; recumbent, isoclinal folds in dolomitic marble, Mosaic Canyon, Death Valley (fingers point to the same layer, which can be traced around a fold hinge to the left); recumbent folds and recumbent folds and extension veins, St. Bude, Cornwall; overturned folds in the Alps; overturned folds with thinning and detachment of the lower limb, Herdla island, near Bergen, western Norway, overturned fold of Fur Fm. (Paleocene) bedrock, Hanklit, Denmark; close-up of recumbent fold; overturned fold, 2; overturned section, Five Springs, Bighorn Mountains]