Here's why:

* Synclines are downward folds: Imagine a bowl shape. The center of the bowl is the lowest point, and in a syncline, the center is the lowest point of the fold.

* Sedimentation: Sedimentary rocks form in layers, with the oldest layers at the bottom and the youngest layers on top.

* Folding: When rock layers are folded into a syncline, the oldest layers are pushed down into the center of the fold, while the youngest layers are on the outside edges.

Therefore, if you were to cut through a syncline, you'd find the oldest rocks in the core of the fold.