General Principles:
* Superposition: In undisturbed rock sequences, the oldest layers are at the bottom, and the youngest layers are at the top.
* Lateral Continuity: Rock layers extend horizontally until they encounter an obstacle.
* Cross-Cutting Relationships: If a feature (like a fault or intrusion) cuts through existing rock layers, it's younger than the layers it cuts.
What We Can Infer:
* Layer Above: The layer above the 120-million-year-old layer is *younger* than 120 million years old. It could be anywhere from slightly younger to significantly younger, depending on how much time has passed since the 120-million-year-old layer formed.
* Layer Below: The layer below the 120-million-year-old layer is *older* than 120 million years old. We can't know its exact age without further investigation.
Important Notes:
* Disturbances: If the rock layers have been disturbed by events like folding, faulting, or erosion, these principles might not apply directly.
* Dating Techniques: To determine the exact ages of the layers, we need to use dating techniques like radiometric dating (e.g., carbon dating, uranium-lead dating).
In Conclusion:
Knowing the age of one layer gives us a starting point for understanding the relative ages of surrounding layers. We can confidently say the layer above is younger, and the layer below is older, but we need more information to determine their precise ages.
