A fold is an undulating structure (wave-like) that forms when rocks or a part of the earth’s crust is folded (deformed by bending) under compressional stress. The folds are made up of multiple strata (rock layers).
The folds that are upwardly convex are called as anticlines. The core (centre) of an anticline fold consists of the older strata, and the strata are progressively younger outwards.
In contrast, the folds that are downwardly convex are called synclines. The core of a syncline fold consists of the younger strata, and the strata are progressively older outwards.