Order Rock Layers and Cutting Events

Use superposition and cross-cutting relationships to reconstruct a sequence.

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In an undisturbed sedimentary sequence, lower layers are older. A feature that cuts an existing rock is younger than the rock it cuts. Check for deformation or missing layers before applying these rules.

Worked example

If an intrusion cuts layers A and B, those layers existed before the intrusion. This establishes relative order, not a numerical age in years.

An Invented Rock Sequence

Undisturbed sedimentary layers lie from bottom to top as A, B, C. An igneous intrusion D cuts all three. No numerical dates are supplied, and no overturning occurred in this model.

1. Which sedimentary layer is oldest? C A D

2. Which layer is youngest among A, B, and C? A B C

3. Is intrusion D older or younger than the layers it cuts? Younger Older Exactly the same age by definition

4. Can these relationships alone establish that C formed exactly 50 million years ago? Yes, the top layer is always 50 million years old No, they give relative order rather than a numerical date Yes, letters encode ages

5. Which observation would require reconsidering a simple lower-is-older assumption? The layers have names A scale bar is printed Evidence that layers were overturned

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