A sandstone in contact with limestone has inclusions of limestone within its basal portion. Which is older?

• A. The sandstone is older than the limestone
• B. The limestone is older and there is an unconformity between the 2 units
• C. They formed simultaneously
• D. The sandstone is younger and there is no contact

Answer: (B)

Inclusions reveal that the material embedded in a rock is older than the surrounding rock that encloses it. Here, limestone fragments are found within the basal portion of the sandstone, so those limestone pieces must have existed before the sandstone formed. As sandstone was deposited, fragments of the preexisting limestone were incorporated at the base, which also indicates there was some erosion before sandstone deposition—a surface representing time gap, i.e., an unconformity, between the two units. Therefore, the limestone is older, and there is an unconformity between the limestone and the sandstone.