What is a nonconformity?

• A. Sedimentary rocks overlie older intrusive igneous rocks or metamorphic rocks.
• B. Intrusive rocks overlie sedimentary rocks.
• C. A fossil bed between igneous rocks.
• D. A surface where rocks create a continuous sequence.

Answer: (A)

A nonconformity happens when sedimentary rocks sit on top of much older crystalline basement rocks (like intrusive igneous or metamorphic rocks) after erosion has removed the upper layers that once covered the basement. This creates a gap in the geologic record where no deposition occurred for a period. The other scenarios don't fit: having intrusive rocks overlie sedimentary rocks is the reverse arrangement, describing a different type of contact; a fossil bed between igneous rocks would not show an eroded surface separating sediment from older rocks; and a surface where rocks form a continuous sequence describes conformity, not a break. So sedimentary rocks overlying older crystalline basement rocks is the clear sign of a nonconformity.