A downward arching rock layer is called syncline.
Explanation:
Syncline and anticline are words used to design folds based on the analogous ages of folded rock layers. A syncline is a fold in which the modern rocks happen in the core of a fold. Where rock layers have been changed prior to folding, such as by folding about a giant fold with a shallowly inclined axial surface, the dearest rocks now occur in the core of folds where layers slide towards the fold axis.
