On a geologic map,if the contacts between sedimentary rock units form a bull's-eye pattern of concentric circles,with the oldest unit in the center,the underlying structure is a(n) ________. 
Definitions:
Copyright Law
A legal framework that gives creators of original works exclusive rights to their use and distribution, typically for a limited time.
Underwriter's Laboratory Mark
A certification mark indicating that a product has been tested for safety by the Underwriters Laboratories (UL).
Trade Symbols
Trade symbols are distinctive marks, logos, or brands used by businesses to identify their products or services uniquely in the market.
American Airlines
A major American airline headquartered in Fort Worth, Texas, known for its extensive international and domestic flight network.
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