Enter layer information and press Calculate Stratigraphy to see modeled ages, occupation phases, and constraint checks.
Stratigraphy Deposition & Occupation Phase Calculator
This calculator helps you turn a recorded stratigraphic sequence into estimated calendar ages for each layer, plus occupation windows based on cultural horizons and TPQ/TAQ constraints. It is designed for field archaeologists, geoarchaeologists, and heritage managers who need a quick depth–age and occupation-phase model while still in the trench or during early analysis.
The tool assumes a one-dimensional vertical section: you describe a stack of layers from the present ground surface downwards, give each layer a thickness, average deposition rate, and any hiatus or erosion event, and optionally flag layers as cultural. From this, the calculator estimates start and end ages for each layer, searches for plausible occupation slices, and checks whether modeled ages are consistent with your Terminus Post Quem (TPQ) and Terminus Ante Quem (TAQ) constraints.
Throughout, “cal year” refers to a continuous calendar-year axis. A common convention is to use positive values for CE/AD years and negative values for BCE/BC (for example, 500 BCE = −500). If your project instead uses calibrated years BP, you can still use the calculator, but you should ensure that all inputs and your interpretation share the same convention and zero-point.
Key inputs and what they mean
The main controls at the top of the form describe the overall time frame and resolution of your model:
- Reference Surface Age (cal year) – The age assigned to the present ground surface at the top of layer 1. Use 0 if you treat the present as year 0, or a specific calendar year (for example 2020) if you want absolute dates. All modeled layer ages are calculated relative to this value.
- Number of Layers – The count of stratigraphic units you want to model. Layers are ordered from the top (1) to the deepest excavated unit (N). This order should follow your field stratigraphic interpretation, not necessarily arbitrary excavation spits.
- Minimum Occupation Slice (years) – The shortest continuous time span that the tool will treat as a distinct occupation phase when aggregating cultural horizons. Very short slices may represent brief visits or palimpsests; longer slices emphasize more substantial occupations.
- Rounding Precision (digits) – The number of decimal places used when reporting modeled ages and durations. Coarser rounding can be more realistic when deposition rates and thicknesses are only known approximately.
For each individual layer, you then provide parameters that control how the age-depth model behaves within that unit:
- Layer Thickness (cm) – The preserved thickness of the layer in centimeters, measured perpendicular to the stratigraphic surface. This is the starting point for estimating how much time that unit represents.
- Rate (mm/yr) and ±% Rate – The average net deposition rate within the layer, expressed in millimeters per year, plus an optional percentage uncertainty. Thicker layers with low rates represent longer timespans, while thin, rapidly deposited layers represent shorter intervals.
- Hiatus After (yr) – A pause in deposition that follows the layer, in years. Conceptually this is a time interval with no net sediment accumulation between this layer and the one above it.
- Erosion Loss (cm) – The amount of the layer that has been removed by erosion, in centimeters. This reduces the preserved thickness and therefore shortens the modeled duration of the unit relative to its original history.
- Cultural? – A flag indicating whether the layer contains cultural material or is part of an occupation horizon. Cultural layers are used to infer occupation phases and to group time slices.
- TPQ and TAQ – Optional chronological constraints for cultural layers. TPQ stands for Terminus Post Quem (the earliest possible date: the occupation cannot be older than this), and TAQ stands for Terminus Ante Quem (the latest possible date: the occupation cannot be younger than this). The calculator compares modeled time ranges for each cultural layer against these bounds and flags inconsistencies.
- Cal Start / Cal End – Output fields that report the modeled start and end ages (in cal years) for each layer, incorporating deposition, hiatuses, and erosion.
How the stratigraphy calculations work
The underlying logic is a deterministic depth–age model. Each layer is treated as having a uniform net deposition rate over its preserved thickness. Time for that layer is approximated by converting thickness from centimeters to millimeters and dividing by the rate:
where thickness is in centimeters, multiplied by 10 to convert to millimeters, and rate is in millimeters per year. This yields a duration t in years for that layer before any erosion adjustment.
If a layer has recorded erosion loss, the model first computes an effective thickness by subtracting the lost centimeters. Only this preserved portion contributes to the modeled duration. Hiatuses are added as discrete blocks of time between successive layers, extending the age difference between their boundaries without adding physical thickness.
Starting from the reference surface age at the top of layer 1, the calculator moves downward, subtracting each layer’s duration and any intervening hiatus from the age to obtain deeper and older horizons. For a layer i, its start and end ages are computed sequentially as:
age at top of layer i = age at top of layer i − 1 − (duration of layer i − 1 + hiatus after i − 1)
age at base of layer i = age at top of layer i − duration of layer i
For cultural layers, the calculator looks at the modeled age range (from cal start to cal end) and compares it to your TPQ/TAQ values. If the modeled range lies entirely older than a TPQ, or entirely younger than a TAQ, the result is flagged as a conflict. Uncertainties in the deposition rate (given by ±% Rate) can be used to generate internal high/low scenarios, but these do not replace formal probabilistic chronological models.
Interpreting layer ages and occupation phases
The main outputs are the modeled start and end ages for each layer, plus any aggregated occupation slices. A layer with a relatively long duration and cultural flag may represent a broad occupation horizon, while several thin cultural layers separated by short hiatuses may be grouped into a single phase if they fall within your chosen minimum occupation slice.
When reading the results:
- Check that age increases (becomes older) with depth in a way that matches your expectations. Non-monotonic sequences may indicate unrealistic rate inputs or misordered layers.
- Note which cultural layers are tightly constrained by TPQ/TAQ and which are effectively unconstrained, relying mainly on the deposition model.
- Look at the duration of each occupation slice relative to your minimum slice setting. Extremely short modeled occupations may be artefacts of your chosen rates and not meaningful archaeological phases.
- Treat exact boundary ages cautiously: they are estimates under simple assumptions, not definitive dates.
Worked example: a simple 4-layer trench
Imagine a small trench with a modern surface over three deeper units. We set the reference surface age to 2020 (cal year) and define four layers from top to bottom:
- Layer 1: 10 cm topsoil, rate 2 mm/yr, no hiatus, no erosion, not cultural.
- Layer 2: 20 cm occupation deposit, rate 1 mm/yr, no hiatus, no erosion, cultural, with TPQ = 1800 and TAQ = 1950.
- Layer 3: 30 cm natural accumulation, rate 0.5 mm/yr, hiatus after = 200 years, no erosion, not cultural.
- Layer 4: 40 cm earlier occupation, rate 0.5 mm/yr, no hiatus, 5 cm erosion loss, cultural, no explicit TPQ/TAQ.
For layer 2, the modeled duration is (20 cm × 10) ÷ 1 mm/yr = 200 years. If the top of layer 2 is estimated around 1900 CE, its base will be around 1700 CE. Because the cultural material is modeled between roughly 1700–1900 CE, and your TPQ/TAQ (1800–1950) overlap that interval, the constraint check should pass, although the upper bound may be tighter than the modeled top.
Layer 3 adds another (30 cm × 10) ÷ 0.5 = 600 years, plus a 200-year hiatus. Layer 4’s preserved thickness is 35 cm (40 − 5), giving (35 × 10) ÷ 0.5 = 700 years of preserved time. Combined, these values push the base of layer 4 well back into the medieval or earlier period, depending on the exact modeled transitions.
Setting the minimum occupation slice to 50 years would yield at least two occupation phases: one modern/post-medieval horizon around layer 2, and an earlier broad phase in layer 4. Adjusting rates or hiatus durations will visibly shift these windows, giving you a sense of how sensitive your interpretation is to field assumptions.
How this tool compares to other stratigraphic approaches
| Method | Main purpose | Strengths | Limitations |
|---|---|---|---|
| This stratigraphy deposition & occupation calculator | Deterministic depth–age and occupation-phase estimates from layer thickness and rates. | Fast, transparent, requires only basic field measurements; ideal for exploratory checks and trench notebooks. | Uses simple average rates per layer; no full probability distributions or calibration curves. |
| Formal Bayesian chronological models | Probabilistic sequences integrating many dated samples and stratigraphic priors. | Rigorous treatment of uncertainty; integrates radiocarbon, OSL, and other datasets. | Requires more data, specialist software, and careful model design; not ideal for quick field assessment. |
| Qualitative stratigraphic interpretation | Narrative description of phases and events based on field observation. | Flexible, can incorporate taphonomic and contextual clues that are hard to quantify. | Lacks explicit time estimates; harder to compare against dated samples or formal models. |
In practice, this calculator is best used alongside your stratigraphic drawings, finds records, and any radiometric dates. It offers a transparent way to test how different assumptions about rates, hiatuses, and erosion affect your inferred occupation phases.
Assumptions and limitations
To interpret the outputs responsibly, it is important to keep the built-in assumptions in mind:
- Uniform rate per layer – Each layer is modeled with a single average net deposition rate. Real deposits often have internal variability that is not represented here.
- One-dimensional vertical model – The section is treated as a simple vertical stack. Lateral variation, complex interfingering, and truncation by features must be interpreted separately.
- Discrete hiatus and erosion events – Pauses in deposition (hiatuses) and erosion losses are modeled as separate steps between otherwise continuous layers. Long-term slow erosion or soil formation is not explicitly represented.
- Model-based, not dated ages – Output ages are model estimates derived from thickness and rates. They are not a substitute for radiocarbon, OSL, or other absolute dating methods, and should not be used alone for high-stakes decisions.
- Cultural flags and occupation slices – Cultural versus non-cultural classifications are entirely user-defined. The algorithm groups cultural horizons into occupation slices mechanically and does not evaluate artefact assemblages or activity types.
- Time scale suitability – The units and rates are most appropriate for typical archaeological to late Quaternary timescales. Very high rates or extremely long durations may produce unrealistic results if extrapolated beyond the context of your data.
Used within these constraints, the calculator can clarify how your field observations translate into a chronological framework and where additional dating or more sophisticated modeling would be most informative.
Thickness-to-Time Modeling for Archaeological Stratigraphy
Translating observed stratigraphic sequences into plausible calendar ages is one of the most challenging tasks in field archaeology and cultural resource management. Excavators often have direct measurements of layer thicknesses, notes on hiatuses, and a handful of diagnostic finds or radiocarbon samples. Yet assembling these disparate clues into a coherent occupational history usually requires spreadsheet gymnastics or slow, bespoke modeling. This calculator streamlines the process by combining measured thickness, deposition rates, erosion allowances, and chronological constraints into a single view. It not only estimates the start and end ages of each layer but also aggregates cultural phases and highlights conflicts with terminus post quem (TPQ), terminus ante quem (TAQ), or calibrated ranges.
The workflow is intentionally pragmatic. Field teams can enter measured thicknesses in centimeters, select reasonable deposition rates in millimeters per year, and flag hiatuses or erosion events. The reference surface age anchors the top of the sequence, so all deeper units automatically shift older (more negative calendar years) as durations accumulate. Because deposition rarely proceeds at a perfectly steady pace, each rate accepts an uncertainty percentage. The calculator propagates that uncertainty into minimum and maximum duration bands, preserving transparency about the plausible age envelopes. Cultural layers can then be grouped into occupation phases, producing composite windows that summarize how long the site was actively used.
Mathematics of Layer Durations
Effective deposition time is calculated by converting net thickness (after erosion) from centimeters to millimeters and dividing by the selected rate. The following MathML expression encapsulates the base equation and the uncertainty propagation used by the calculator:
Rates entered as zero or negative are automatically rejected, while uncertainties greater than 100% are clipped to preserve meaningful bounds. If erosion equals or exceeds thickness, the effective duration collapses to zero, which the results note explicitly. Hiatus durations are treated as additive gaps after each layer, pushing older layers further back in time without altering the duration of the layer itself. This approach matches how archaeologists narrate sequences in reports: “After the abandonment of US 103 there was a 40-year hiatus before the deposition of US 104.”
Occupation Phases and Cultural Aggregation
Stratigraphic profiles often mix sterile depositional units with cultural surfaces, hearths, or construction episodes. To better understand human activity, the calculator merges consecutive cultural layers into broader occupation phases. Each phase captures the youngest boundary from the uppermost cultural layer and the oldest boundary from the deepest cultural layer in the run. The total occupation duration sums the modeled times of all cultural layers. The MathML snippet below expresses this aggregation:
Phases aid in reporting because they condense multiple stratigraphic units into a single occupational story. Rather than listing five thin living surfaces individually, a CRM report can state that “Occupation Phase 2 spans 120 to 60 cal BCE with a best-estimate duration of 60 years.” The calculator automatically performs this roll-up while preserving per-layer details and total cultural versus sterile time slices.
Understanding Hiatuses and Erosion
Hiatus durations represent pauses in deposition or occupation. They may reflect abandonment episodes, natural stabilization, or unexcavated intervals. Entering a hiatus after a layer shifts all deeper layers to older ages by the specified number of years. Erosion losses, by contrast, reduce the effective thickness of a layer. When a profile shows that the upper part of a deposit has been truncated, subtracting that thickness avoids overestimating the duration of the surviving portion. The calculator prevents erosion from exceeding the recorded thickness and highlights layers where the adjustment yields zero time. In practice, archaeologists interpret hiatuses cautiously: anything longer than a generation (20–30 years) usually leaves some surface development or cultural markers, while shorter gaps might reflect seasonal pauses.
Constraints: TPQ, TAQ, and Calibrated Ranges
Chronological constraints anchor stratigraphic estimates. A terminus post quem (TPQ) indicates the earliest possible time a deposit could have formed, typically derived from diagnostic artifacts or radiocarbon assays. If modeled ages fall entirely before the TPQ, the result conflicts with the physical evidence. A terminus ante quem (TAQ) provides the latest permissible age, often from overlying structures or dated events. Calibrated date ranges give a probabilistic window for a sample. The calculator checks whether the modeled interval overlaps the provided range and flags any layers that sit wholly outside. When conflicts emerge, archaeologists can adjust deposition rates, hiatus assumptions, or re-examine the stratigraphic interpretation.
Worked Example Using the Default Values
The default dataset depicts a six-layer sequence anchored at the present (0 cal year) with a mix of cultural and sterile units. Layer 1 (US 100 Topsoil) is eight centimeters thick with a deposition rate of 1.8 mm/yr and an uncertainty of 20%. After accounting for a 25-year hiatus, the younger boundary remains at 0 cal year, while the older boundary shifts to approximately −44 years with min/max brackets of −37 to −55 cal years. Because it is sterile, the occupation duration contributes zero.
Layer 2 (US 101 Floor) is five centimeters thick, deposited at 0.9 mm/yr with a 15% uncertainty and no hiatus. The calculator subtracts the duration from the inherited boundary of −44 cal years, yielding an older boundary around −100 cal years. Since the layer is marked cultural, its roughly 56-year duration contributes entirely to occupation time. The provided TPQ of −120 cal years fits comfortably within the modeled interval, and the calibrated range from −150 to −30 cal years overlaps the estimate, so the constraint status is OK.
Layer 3 (US 102 Levelling Fill) thickens to fourteen centimeters at 1.1 mm/yr and includes a 40-year hiatus afterward. Its best estimate spans about 127 years, moving the boundary from −100 to roughly −227 cal years before the hiatus pushes subsequent layers even older. Because it is sterile, the duration adds to the sterile total. Layer 4 (US 103 Hearth Lens) is a thin cultural layer three centimeters thick with a slow deposition rate of 0.6 mm/yr. Its 50-year modeled duration falls between −227 and −277 cal years, comfortably matching the associated radiocarbon interval.
Layer 5 (US 104 Collapse) includes an erosion loss of two centimeters out of eighteen. The effective sixteen-centimeter thickness at 1.4 mm/yr yields about 114 years. A 60-year hiatus afterward creates a notable gap in the sequence. Because this layer is non-cultural, it adds to sterile time. Finally, Layer 6 (US 105 Sterile Clay) accumulates 25 centimeters at 0.7 mm/yr with 20% uncertainty, producing a duration of roughly 357 years and pushing the base of the excavated sequence to around −808 cal years. The cumulative occupation time sums the cultural durations of layers 2 and 4, while sterile time includes the other four layers plus all hiatus periods when calculating totals.
The results table also reports min and max ages. For example, if the deposition rate of the hearth lens were 25% faster than the best estimate, the occupation could compress to roughly 40 years, while a 25% slower rate would extend it beyond 65 years. These ranges help archaeologists gauge whether their interpretive narrative remains plausible across reasonable rate variations.
Scenario Comparisons
Adjusting deposition rates or hiatus values reveals how sensitive the timeline is. Suppose we increase all cultural layer rates by 20% while keeping sterile layers unchanged. The total occupation duration shrinks, and the phase windows tighten. Conversely, adding a 30-year hiatus after Layer 2 creates a noticeable gap that might correspond to temporary abandonment. The tables below compare three scenarios: the default inputs, accelerated cultural rates, and an added hiatus.
| Scenario | Total Occupation (yr) | Total Sterile (yr) | Total Hiatus (yr) | Oldest Boundary (cal yr) |
|---|---|---|---|---|
| Default | ≈106 | ≈598 | 125 | ≈−808 |
| Faster Cultural Rates (+20%) | ≈88 | ≈598 | 125 | ≈−790 |
| Added 30-year Hiatus after Layer 2 | ≈106 | ≈598 | 155 | ≈−838 |
A second comparison explores uncertainty bands. If the uncertainty for Layer 5 increases from 35% to 80%, the max duration nearly doubles, dragging the basal age much older while the minimum remains stable. High uncertainty is useful when little is known about deposition rates, but the expanded window should be clearly communicated in reporting.
| Layer | Uncertainty (%) | Duration Min (yr) | Duration Best (yr) | Duration Max (yr) |
|---|---|---|---|---|
| US 104 Collapse (Baseline) | 35 | 84 | 114 | 176 |
| US 104 Collapse (High Uncertainty) | 80 | 71 | 114 | 285 |
Practical Caveats
Rate-based modeling assumes consistent accumulation within each layer, yet natural deposits often fluctuate due to seasonal floods, anthropogenic dumping, or bioturbation. Cultural surfaces can build up rapidly during intense occupation and then remain stable for years. Compaction may shrink the observed thickness relative to the original deposit, especially for organic-rich layers. Re-deposition can mix older materials into younger contexts, complicating TPQ and TAQ logic. The calculator cannot resolve these complexities automatically, but it makes the assumptions explicit so archaeologists can defend or revise them.
Another caveat involves lateral variability. A layer that thins toward the edge of a trench may reflect localized erosion or slope wash. Entering a single thickness value assumes the measured point is representative. Teams can run multiple scenarios using minimum and maximum observed thicknesses to understand the envelope of possibilities. Similarly, variable sedimentation within a layer might warrant splitting it into sublayers with different rates.
Differential preservation is also critical. If post-depositional processes removed the top of a cultural layer, the remaining thickness underestimates occupation duration. Adding an erosion loss approximates this effect, but the user must estimate how much material is missing. When in doubt, pairing this calculator with micromorphology studies or additional dating samples can refine interpretations.
Adapting the Model
Some researchers may prefer separate deposition rates for cultural versus sterile layers. The calculator already supports this by allowing per-layer rate entries. For more sophisticated modeling, users can export the results (via the copy button) and feed them into Bayesian chronological frameworks such as OxCal or ChronoModel. The min and max ages provided here can serve as priors or plausibility checks. Another adaptation is to treat occupation duration as a minimum slice: the “Minimum Occupation Slice” input lets teams specify a floor for cultural durations, rounding very thin surfaces up to a reasonable interpretive unit, such as a single season or year.
Frequently Asked Questions
How should I choose deposition rates?
Rates can be derived from local studies, micromorphology, or published analogues. Urban trash pits might accumulate at 5–10 mm/yr, while natural alluvium could build at 0.5–2 mm/yr. When uncertain, enter a best guess with a generous uncertainty percentage to reveal how much the timeline could vary.
What if I only know the calibrated date range?
Enter the range start and end. The calculator checks whether the modeled interval overlaps the range. If the layer falls entirely outside, the status flags “Needs Review” so you can re-examine assumptions or consider re-dating.
Can hiatus durations represent occupation?
Hiatuses simply add gaps between layers. If you believe people occupied the site during a hiatus, consider modeling that time as a thin cultural layer with minimal thickness instead of a hiatus. This preserves the distinction between human activity and sedimentation pauses.
How do BCE and CE years work?
Negative numbers represent BCE (e.g., −500 = 500 BCE), while positive numbers represent CE. The calculator maintains these conventions across inputs and results. When summarizing results, specify the sign or convert to BCE/CE notation for clarity.