Determining Relative Ages

Take a look at this geologic cross-section. It has three sedimentary layers labeled A, B, and C, an igneous intrusion labeled D that formed when magma solidified, and a fault labeled E.

Let's see how we can determine the relative ages of these features.

According to the law of superposition, the oldest layers are formed on the bottom, and new rock layers are always deposited on the top. Since layer C is the bottom one, it must have formed first, followed by the middle layer B and the top layer A.

Now, observe the igneous intrusion labeled D cutting through all three layers. According to the principle of cross-cutting relationships, an intrusion or fault is younger than the rock layers it cuts through. By applying this, we can deduce that intrusion D is younger than layers A, B, and C.

Finally, fault E shifts everything, including the intrusion, making it the youngest feature.

We can also see a layer of soil over layer A, the final event in this cross-section, formed by weathering and erosion.