On a geologic map, if the contacts between sedimentary rock units form a series of parallel lines, with the youngest unit in the center, the underlying structure is a .

Syncline

Explanation:A syncline is a fold in which the youngest rocks occur in the core of a fold (i.e., closest to the fold axis), whereas the oldest rocks occur in the core of an anticline.

A fold of rock layers that slope upward on both sides of a common low point. Synclines form when rocks are compressed by plate-tectonic forces. They can be as small as the side of a cliff or as large as an entire valley.

Synclines are formed when tectonic plates move toward each other, compressing the crust and forcing it upward.

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