Look at a geologic cross section of the earth and understand how to determine the relative ages / sequence of events.

The geologic cross section below, illustrates a series of rock layers as they might appear in a roadcut or quarry wall. The rock layers and events are lettered. In order to determine the relative ages we use the four fundamental principles of relative age dating that were explained to you on the concepts page.

Geologists think about cross sections of the earth like the side view of a layer cake. In general, the oldest units are on the bottom and the youngest units are on the top (law of superposition). There can be lots of complexities, such as folding events, faulting, erosion events, intrusion by magma, etc. We put these events into the order in which they occurred, starting with the oldest, and working toward the youngest. Recall the law of cross-cutting relationships to figure out "what cuts what". If a fault cuts a rock layer, then the rock layer is older than the fault.

For example in the diagram below, the sedimentary units are in sequence, P, K, M, S, and X. Then something happened. A body of magma (INTRUSION R) has intruded or cut through all of the previous layers. So it comes next in the sequence. The intrusion is eroded off at the top. (The previous layers are eroded off at the top too.) So the event after the intrusion is EROSION A. After EROSION A, beds B, J, and F were deposited. So your answer for the relative ages, or sequence of events, would be: P, K, M, S, X, intrusion R, erosion A, B, J, F.

Now that you've followed the steps involved to determine the relative ages of the rock layers in the above cross section, try your hand at putting the rock layers in sequence yourself. Examine the geologic cross section below, and determine the relative ages of the lettered rock bodies and features such as faults or surfaces of erosion. In order to determine the relative ages use the four fundamental principles of relative age dating that were explained to you on the concepts page.  Always start with the oldest rock and work toward the present.

Once you've figured out the relative ages, type your answers in the white blocks, then go to the bottom of the page and click on the button marked "Submit Answers for Correction." A box will open which will tell you the answers you missed. Click OK and it will give you the number correct, the number left blank, and the number wrong.

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