Nonconformity can be described as which of the following?

• A. A type of unconformity where layered sedimentary rocks lie on an erosion surface cut into igneous or metamorphic rocks
• B. A cross-cutting fault
• C. A surface where sedimentary rocks overlie igneous or metamorphic rocks
• D. A surface indicating continuous deposition

Answer: (A)

Nonconformity is an unconformity where a surface of erosion separates younger sedimentary rocks from older crystalline or metamorphic basement rocks. The essential idea is that there was a period of erosion that removed previously formed rocks, exposing igneous or metamorphic material, and later sedimentary rocks were deposited on top. That’s why the best description is layered sedimentary rocks resting on an erosion surface cut into igneous or metamorphic rocks—the erosion surface marks the break in time and the boundary between the two rock types.

A surface where sedimentary rocks overlie igneous or metamorphic rocks can describe the boundary, but it doesn’t explicitly include the erosion surface that defines a nonconformity. A cross-cutting fault would be a structural feature cutting through rocks, not a surface reflecting a depositional gap. A surface indicating continuous deposition would imply no unconformity at all.