In sandstone containing limestone fragments at its base, which statement best describes the age relationship?

• A. The sandstone is older
• B. They formed in the same event
• C. The limestone is older and there is an unconformity
• D. There is no relation

Answer: (C)

When older rock fragments appear inside a younger sedimentary unit, they show the source material came from rocks that existed before the younger deposition. If limestone fragments are found at the base of a sandstone, the limestone must be older than the sandstone because those clasts had to form first and then be incorporated into the later sandstone. The placement at the base also suggests an erosion or non-depositional interval between the limestone and the sandstone, i.e., an unconformity. So the limestone is older, and there is an unconformity between the two units.