Principles of Stratigraphy
Stratigraphy is the study of layered deposits (called strata) and the order in which they formed. By reading these layers from bottom to top, archaeologists can reconstruct the sequence of events at a site without ever needing an absolute date. It's one of the most fundamental tools for building a relative chronology, which means figuring out what happened before and after something else, even if you don't know the exact year.
Principles of Stratigraphy
Concept of Stratigraphy
At its core, stratigraphy treats the ground beneath a site as a record of everything that happened there. Sediment, debris, ash, and soil accumulate over time, and each distinct layer reflects a specific period or event.
- The oldest material sits at the bottom; the youngest sits near the surface
- Each layer can contain artifacts, ecofacts, or structural remains that belong to the time period when that layer formed
- By tracking changes across layers, archaeologists can observe cultural shifts, such as the transition from hunter-gatherer lifeways to agricultural societies
This makes stratigraphy essential for giving artifacts context. A stone tool means something very different depending on whether it comes from a deep, early layer or a shallow, recent one.
Law of Superposition
The law of superposition states that in any undisturbed sequence of layers, the lowest layer was deposited first and is therefore the oldest. Each layer above it is progressively younger.
This principle lets archaeologists establish relative dates:
- A pottery shard found in a lower layer is older than a pottery shard found in an upper layer
- A stone tool buried deep in the sequence predates features found closer to the surface
The key qualifier is undisturbed. If layers have been mixed by plowing, burrowing animals, or later construction, the law of superposition may not hold, and archaeologists have to account for that disruption.
The law also provides a framework for tracking cultural transitions at a site. For example, if Paleolithic stone tools appear in the lowest layers and Neolithic ground-stone tools appear higher up, you can infer the sequence of cultural periods even without radiocarbon dates.
Principles of Stratigraphic Relationships
Three additional principles help archaeologists interpret what they see in the ground:
- Original horizontality: Layers of sediment are deposited in a roughly horizontal position. If you find layers that are tilted, folded, or warped, something disturbed them after they were laid down (tectonic activity, slumping, or human modification).
- Lateral continuity: A layer extends continuously in all directions until it naturally thins out or hits the edge of the area where deposition was occurring. This means you can trace the same layer across different parts of a site or even correlate it across a region. A volcanic ash layer, for instance, can serve as a time marker across multiple sites.
- Cross-cutting relationships: Any feature that cuts through an existing layer or feature is younger than what it cuts through. This is how you determine the relative age of things like pits, ditches, and walls. If a storage pit was dug through an existing floor surface, the pit is younger than the floor.
Types of Stratigraphic Units
Not every deposit looks the same. Archaeologists distinguish between several types of stratigraphic units:
- Layers (strata): Distinct, relatively uniform bands of sediment or soil deposited horizontally across a site. Each layer represents a specific period or event, such as a flood deposit, a phase of occupation, or a destruction episode.
- Lenses: Small, lens-shaped deposits found within a larger layer. These typically represent localized, short-term activities. A concentration of charcoal and ash from a hearth, or a small dump of food waste, would form a lens rather than a full layer.
- Features: Non-portable elements of a site that are either built (walls, buildings), excavated into the ground (pits, ditches), or naturally formed (postholes, root disturbances). Features provide direct evidence of how people organized and used space, from storage pits to burial sites.
Interpreting Stratigraphic Sequences
Using Stratigraphic Sequences to Reconstruct Past Events and Environments
A stratigraphic sequence is essentially a timeline preserved in the ground. Reading it involves working from the bottom (oldest) to the top (most recent) and identifying what each layer or feature represents.
Archaeologists use these sequences to:
- Track changes in human activity over time. A shift from layers rich in animal bone to layers containing carbonized grain seeds can signal a transition from hunting to farming.
- Reconstruct past environments. Pollen, sediment type, and soil chemistry within layers can reveal climate change, deforestation, or flooding events.
- Identify episodes of occupation, abandonment, or destruction. A thick sterile layer (no artifacts) between two occupation layers suggests the site was abandoned for a period. A burn layer might indicate a fire or conflict.
Stratigraphic sequences also become more powerful when correlated across multiple sites in a region. If a distinctive pottery style or a volcanic ash layer appears at the same stratigraphic position in several sites, it ties those sites together chronologically. This kind of correlation helps archaeologists map broader patterns like trade networks, migration routes, and shared cultural traditions.