What term describes the plane that divides folded rock layers into two halves?

• A. Axial plane
• B. Hinge line
• C. Fault plane
• D. Cleavage plane

Answer: (A)

In folded rocks, the surface that acts as the mirror line for the fold is the axial plane. This plane runs through the hinges of the beds and demarcates the two halves of the fold, essentially serving as the plane of symmetry. The hinge line is a line along which the fold reaches maximum curvature in a given bed, so it’s a line, not a plane. A fault plane describes a surface where rocks break and slide past one another, related to tectonic movement rather than the geometry of a single fold. A cleavage plane is a directional fabric in metamorphic rocks, indicating mineral alignment, not the dividing surface of folded layers. Therefore, the plane that divides folded rock layers into two halves is the axial plane.