Units A, B, C, D, and F are sedimentary rocks. Unit E is an igneous rock.

-Which is the oldest unit?

Question

Units A, B, C, D, and F are sedimentary rocks. Unit E is an igneous rock.
 
-Which is the oldest unit?

Quizplus · Accepted Answer

The principle of superposition states that in a series of sedimentary rock layers, the oldest layer is at the bottom and the youngest is at the top. Since all of the units are sedimentary except for E, which is igneous, we cannot use superposition to determine the age relationships between the units. However, we know that igneous rocks are always younger than the rocks they intrude or cut across. Unit E is an igneous rock, so it must be younger than all of the sedimentary units. Therefore, C is the oldest unit.

Units A, B, C, D, and F Are Sedimentary Rocks