In an area with limited surface outcrops, a large overturned anticline composed of an incompetent bed bounded by two competent beds. How would you determine which limb is overturned?

• A. Examine drag folding within the incompetent bed
• B. Examine strike differences between limbs
• C. Check bedding thickness
• D. Look for fossil assemblages

Answer: (A)

When a large overturned anticline is bounded by two competent beds with a weak, incompetent bed in between, the soft layer records how the limbs move during folding. As the anticline twists, the rigid boundaries resist bending, and the incompetent bed gets dragged and sheared between them. This creates drag-like folds within the soft bed that align with the direction of movement. By examining the incompetent bed for pronounced drag features, you can identify which limb has been flipped over—the limb toward which the soft bed is dragged is the overturned one.

Other signs are less reliable here. Differences in strike don’t directly reveal overturning direction, and changes in bedding thickness can arise from original deposition or later effects rather than overturning. Fossil assemblages could help in some cases, but with limited surface exposure they’re not as dependable for locating the overturned limb.