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How does stratigraphy help archaeologists establish the order of occupation at a site?

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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.

In archaeology, stratigraphy serves as a fundamental method of relative dating, allowing researchers to determine the chronological order of occupation at a site by observing the succession of accumulated deposits.1

The primary principle guiding this process is the law of superposition, which states that in any undisturbed sequence of layers, the oldest deposits are located at the lowest level, while progressively younger materials are found in the layers above. Archaeologists apply this by analyzing the vertical accumulation of debris—such as that found in settlements, burial mounds, or pits—to establish a relative chronology of different occupation levels.2341

It is important to note that depth alone is not always a reliable indicator of age, as site formation processes can be complex. Archaeologists must carefully distinguish between layers to ensure that artifacts found within the same level are truly contemporary and not the result of later intrusions, such as burials or disturbances. By observing these distinct layers, researchers can reconstruct the sequence of human activity at a site relative to other levels.1