Principles and Techniques of Relative Dating
Relative dating tells you whether something is older or younger than something else, not how old it actually is. That distinction matters because most archaeological sites lack the materials needed for absolute dates. Stratigraphy, seriation, typology, and cross-dating are the core toolkit for building these relative chronologies.
By analyzing how layers, artifacts, and styles relate to each other, archaeologists can reconstruct sequences of events and cultural change across sites and regions.
Principles of Stratigraphy in Archaeology
Stratigraphy is the study of layered deposits, called strata, to establish a relative chronology at a site. It's the most fundamental relative dating method.
The key principle is the Law of Superposition: in an undisturbed sequence, the lowest layers were deposited first and are therefore the oldest, while layers above them are progressively younger. So if you find pottery in a deep layer and a metal tool in a layer above it, the pottery is relatively older.
Stratigraphic relationships also help identify events that disrupted the original layering:
- Intrusive features like pits, postholes, or burials cut through existing layers. The intrusion is always younger than the layers it disturbs.
- The fill inside an intrusive feature (the soil and debris that accumulated after it was dug) is younger than both the feature itself and the surrounding strata.
To manage the complexity of real excavations, archaeologists use a tool called the Harris Matrix. This is a diagram that maps out all the stratigraphic relationships at a site, showing which layers and features are older, younger, or contemporary with each other. It has been applied at sites with dense, complicated sequences like Pompeii and Çatalhöyük.
Seriation for Chronological Sequencing
Seriation arranges artifacts into chronological order based on how their attributes change over time. The core assumption is that artifact styles shift gradually and in a consistent direction. Pottery decoration, for example, doesn't jump randomly from one style to another; it evolves.
There are two main approaches:
- Frequency seriation looks at how common different artifact types are across multiple assemblages. If a particular pottery style makes up 5% of one assemblage, 40% of another, and 10% of a third, you can arrange those assemblages in a sequence that reflects the style's rise and fall in popularity. This produces the classic "battleship curve" pattern when graphed.
- Contextual seriation factors in the stratigraphic and spatial context of artifacts within a site. This helps refine the chronological sequence and identify which assemblages were actually in use at the same time.
Seriation is particularly useful at prehistoric sites and early civilizations where absolute dating methods can't easily be applied. It gives archaeologists a way to order materials even when no calendar dates are available.
Typology in Relative Dating
Typology classifies artifacts into distinct types based on shared physical attributes like shape, size, material, and decoration. The underlying assumption is that artifacts of the same type were made during the same general time period or belong to the same cultural tradition. Projectile points and ceramic vessels are two of the most commonly used artifact categories for typological analysis.
By arranging types into sequences, archaeologists can trace how material culture changed over time and build relative chronologies for a region.
Typology has real limitations, though:
- Some artifact types persist for very long periods or get reused and recycled, which can make them appear contemporaneous with later materials when they aren't.
- Regional variations and cultural differences mean that a typological sequence developed for one area may not apply to another.
- Typology on its own cannot produce absolute dates. It needs to be combined with other methods to anchor sequences to actual time periods.
Cross-Dating for Site Correlation
Cross-dating connects findings from different sites by comparing artifacts, features, or layers across locations. The goal is to establish that certain deposits or objects at separate sites are roughly contemporaneous.
There are several ways to cross-date:
- Stratigraphic cross-dating correlates strata between sites based on their relative position and the diagnostic artifacts found within them. This requires careful, detailed comparison of each site's stratigraphic sequence.
- Typological cross-dating compares artifact types and styles across sites. If two sites share the same distinctive pottery style, for example, the layers containing that pottery are likely from the same general period. Trade goods and architectural styles are especially useful here because they spread across regions.
- Anchoring with absolute dates uses methods like radiocarbon dating, or historical sources such as written records and coins, to pin relative sequences to actual calendar dates. For instance, finding a Roman coin of known date in a stratigraphic layer gives that layer an approximate absolute date, which then helps date the layers above and below it.
Cross-dating has been applied extensively in regions with networks of related sites, such as Mayan centers in Mesoamerica and Roman provincial settlements across Europe.