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Why is greater depth below ground not always proof that an artifact is older?
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Archaeology
Archaeologists mapping their finds at Pachacamac, Peru, an Indigenous town occupied from approximately 200 bce to 1532 ce, when it was sacked by conquistadors under the command of Francisco Pizarro.
While the law of superposition generally dictates that in undisturbed deposits, older artifacts are found in lower layers, greater depth is not always proof of greater age. Several factors can disrupt this stratigraphic sequence:
- Bioturbation: This is the restructuring of sedimentary deposits by living organisms, such as worms or burrowing animals, which can move artifacts between different soil layers.1
- Geological Deformation: Natural processes can cause rock layers to tilt or even overturn, potentially placing older material above younger material.2
- Erosion: The removal of surface material can blur the stratigraphic record, making it difficult to distinguish the original top and bottom of a sequence.2
- Human Activity: Archaeological sites are often subject to human disturbance, such as digging, construction, or the creation of pits and trenches, which can displace artifacts from their original context.
Because of these potential disturbances, archaeologists must rely on careful recording and, when necessary, additional dating methods—such as radiometric dating—to accurately interpret the age of recovered remains.34