How do you identify the oldest layer?

To identify the oldest layer in a geological context, you can use the principle of superposition, which states that in undisturbed sedimentary rock layers, the oldest layers are at the bottom, while the younger layers are deposited on top. Additionally, radiometric dating techniques can provide precise ages for specific layers. Fossil content, through biOStratigraphy, can also help correlate and date layers, indicating which are older based on the fossils they contain.