Here's why:

* Synclines are downfolds: They are troughs or dips in the rock layers, shaped like a "U" or a bowl.

* Rock layers are deposited horizontally: Sedimentary rocks are originally laid down in flat, horizontal layers.

* Folding distorts the layers: When the Earth's crust is compressed, these horizontal layers bend and fold.

* Oldest rocks are at the bottom: The oldest rocks in a sequence were deposited first and will therefore be found at the bottom of the original sequence.

Therefore, since the syncline fold is a downfold, the center of the fold will contain the bottommost and oldest rock layers of the sequence.