In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds.[1] Synclines are typically a downward fold, termed a synformal syncline (i.e. a trough); but synclines that point upwards can be found when strata have been overturned and folded (an antiformal syncline).
On a geologic map, synclines are recognized by a sequence of rock layers that grow progressively younger, followed by the youngest layer at the fold's center or hinge, and by a reverse sequence of the same rock layers on the opposite side of the hinge. If the fold pattern is circular or elongate circular the structure is a basin. A notable syncline is Wyoming's Powder River Basin. Folds typically form during crustal deformation as the result of compression that accompanies orogenic mountain building.
A very well exposed syncline is at the Sideling Hill roadcut on Interstate 68 in western Maryland, USA, where the Rockwell Formation and overlying Purslane Sandstone are exposed.
A spectacular example of a perched syncline, the highest in Europe, is Saou, in the Alpine foothills of south-eastern France.