Overview and when to use it
This calculator turns a recorded stratigraphic sequence into estimated calendar ages for each layer and then summarizes potential occupation phases inside that sequence. It is designed for archaeologists, geoarchaeologists, heritage consultants, students, and field teams who want a transparent first-pass model while they are still standing in the trench, checking context sheets, or sketching an early site narrative. Rather than hiding the chronology inside a black box, the tool makes each assumption visible: thickness, average deposition rate, uncertainty, hiatus, and erosion are all entered layer by layer so you can see exactly why the timeline shifts.
The page assumes you are describing a vertical stack of deposits from the present ground surface downward. Layer 1 sits at the top of the section, and each deeper layer is treated as older unless you enter a hiatus or a constraint that suggests a conflict. You can also mark layers as cultural, which allows the results to group consecutive cultural deposits into occupation phases. That feature is especially useful when you are moving from individual context descriptions toward a broader statement such as intermittent occupation, a sustained use horizon, or a buried activity surface separated by sterile accumulation.
Calendar years are handled as a continuous numeric axis. A common convention is to use positive numbers for CE or AD and negative numbers for BCE or BC, so 500 BCE becomes -500. If you work in cal BP or another chronological system, that is fine too; the important point is consistency. The reference age, TPQ, TAQ, and any calibrated start or end ranges all need to use the same year scale, otherwise the conflict checks will be misleading even when the stratigraphy itself is sensible.
Entering your sequence
Start with the reference surface age, which is the age assigned to the top of Layer 1. In many fast field uses you may enter 0 to represent present equals zero. In a historical or absolute chronology workflow you might instead enter 2020, 1950, or another anchor year that matches your project convention. The next choice is the number of layers. As soon as you change that count, the table re-renders so you can enter a simple three-layer sequence or a much more detailed excavation profile with many units.
Each row asks for thickness in centimeters, deposition rate in millimeters per year, rate uncertainty as a percentage, hiatus after the layer in years, erosion loss in centimeters, and optional chronological checks. The cultural checkbox does not change the age math; it changes how the results describe occupation. If a cultural layer is extremely thin, the main result still calculates its duration from thickness and rate, but the occupation summary can enforce a minimum occupation slice so that very thin activity lenses do not collapse to an unrealistic near-zero occupation span in your report text.
- Set the reference surface age. This anchors the top boundary of Layer 1.
- Choose the number of layers. Layers are always ordered from top to bottom.
- Enter thickness, rate, uncertainty, hiatus, and erosion. Thickness and erosion use cm; rate uses mm per year.
- Flag cultural layers where appropriate. These are the layers the calculator uses for phase grouping.
- Add TPQ, TAQ, or calibrated ranges if you have them. They are treated as checks, not forced constraints.
- Adjust minimum occupation slice and rounding. These settings affect reporting, not the physical layer order.
- Calculate and review warnings carefully. The notes section is often as informative as the numbers themselves.
A few practical input habits make the results much easier to trust. Enter measured thickness and estimated erosion separately rather than folding them together in your head. Keep the rate field for a net average accumulation rate within the layer, not for a single event pulse unless the deposit truly represents one. If a hiatus is meant to sit between Layer 3 and Layer 4, place it in the Hiatus After field for Layer 3. That detail matters because the calculator applies the time gap after the layer you enter it on.
How the model turns thickness into time
The core logic is intentionally simple. First, the tool calculates effective preserved thickness by subtracting erosion from recorded thickness. It then converts centimeters to millimeters and divides by the deposition rate. The result is a modeled duration for that layer. If you provide rate uncertainty, the calculator also produces a faster-accumulation and slower-accumulation bracket so you can see minimum and maximum duration scenarios without changing the best estimate. That is useful when you know your rate is plausible but not tightly constrained.
Hiatus works differently from thickness. A hiatus adds time without adding deposit. That means a long hiatus can make everything below it substantially older even if the layers beneath are thin. Erosion works in the opposite direction: it removes preserved thickness, so the same apparent unit can contribute fewer modeled years than you would expect from its full original deposit. These two adjustments are why the tool is useful for more than a simple thickness divided by rate calculation. It helps separate preserved sediment from missing time.
A short worked example shows the logic. Suppose the reference surface age is 2020. Layer 1 is 10 cm thick and accumulates at 2 mm per year, with no erosion and no hiatus. Its duration is 50 years, so its base falls at about 1970. If Layer 2 is 20 cm at 1 mm per year, its duration is 200 years, so its base falls at about 1770. If there is then a 200-year hiatus after Layer 2, the top of Layer 3 becomes about 1570 before Layer 3's own duration is applied. In other words, the gap pushes the deeper sequence older even though the hiatus itself has no thickness.
The optional TPQ, TAQ, and calibrated start or end fields are not used to recalculate the ages. Instead, they are used to ask whether your modeled interval overlaps the evidence you already have. If a layer modeled between -320 and -260 is supposed to contain a find that gives a TPQ of -180, the calculator can flag that as a conflict because your model leaves the layer entirely earlier than the earliest permitted date. That kind of warning is often the moment when a site narrative improves: perhaps the unit needs to be split, perhaps the rate is too slow, perhaps the marker is residual, or perhaps the stratigraphic assumption needs another look.
Reading the output
The main results table lists effective thickness, rate and uncertainty, duration, younger boundary, older boundary, occupation versus sterile contribution, hiatus, and constraint status for each layer. Younger boundary means the top of the layer, closer to the surface and usually later in time. Older boundary means the base of the layer, deeper in the section and earlier in time. If uncertainty is entered, each boundary is shown as best, minimum, and maximum, which gives you a compact sensitivity view without forcing a separate scenario run.
Occupation phases are generated mechanically from consecutive cultural layers with non-zero modeled duration. That is a helpful summary, but it is still a summary of your inputs rather than a substitute for interpretation. A run of cultural deposits might represent continuous use, repeated resurfacing, or several short episodes blurred together by excavation resolution. The calculator simply reports that the cultural layers are consecutive and provides the modeled span from the youngest boundary of the uppermost cultural layer to the oldest boundary of the deepest cultural layer in the run.
When you interpret the totals, pay close attention to the balance among occupation years, sterile years, and hiatus years. A deep sequence with little preserved sediment can still represent a long chronological span if hiatuses dominate. Conversely, a thick sterile fill can produce many modeled years even if it contains no cultural signal. That is why the totals should be read alongside the layer-by-layer table rather than in isolation.
- Monotonicity matters. Boundaries should generally get older with depth. If they do not, re-check layer order, rate entries, and hiatus placement.
- Large hiatuses are powerful. They shift every deeper layer older without adding thickness.
- Erosion can shrink durations dramatically. If erosion equals thickness, the layer contributes essentially no preserved time.
- Constraint status is a guide, not a verdict. OK means no obvious contradiction, Needs Review usually signals range mismatch, and Conflict means the modeled interval is inconsistent with the supplied bound.
In day-to-day practice, this tool is most valuable as a structured plausibility check. Before you spend time building a detailed Bayesian chronology or writing a final phasing narrative, you can ask whether the layer thicknesses, rates, hiatuses, and constraints tell a coherent story. If they do not, the transparent arithmetic here often makes the weak assumption visible very quickly.
Assumptions, limits, and field judgement
This calculator uses one average net deposition rate per layer. That means it does not model internal variation such as a rapid dump followed by slow soil formation within the same unit. It also treats the sequence as a one-dimensional stack. Lateral changes, interfingering deposits, complex cuts, and strongly time-transgressive units need more careful treatment than a single vertical depth-age model can offer. None of those limitations make the tool unhelpful; they simply define what kind of question it is best at answering.
Rate choice is often the least certain part of the exercise. If you have local dated analogues, micromorphological evidence, or known sedimentation studies, use them. If you do not, choose a defensible best estimate and express uncertainty honestly. A field estimate of 1.0 mm per year with 50 percent uncertainty tells a clearer and more responsible story than a false appearance of precision. Likewise, when you have strong dated markers such as a floor associated with a documented event, a sealed coin, a tephra, or a calibrated radiocarbon range, use the TPQ, TAQ, and calibrated fields to test your model rather than ignoring that independent evidence.
For publication or compliance work, the output should usually be described as a scenario or a deterministic screening model, not as the final chronology. It is excellent for trench-side reasoning, rapid consistency checks, method notes, and explaining why one rate assumption produces a more believable phase span than another. It is not a replacement for direct dating, specialist interpretation, or full chronological modeling when precision matters.
- Reference surface age
- The calendar age assigned to the top of Layer 1. Every deeper boundary is calculated relative to this anchor.
- Effective thickness
- The preserved thickness after subtracting erosion loss. This is the thickness used in the duration formula.
- Hiatus
- A period of no net deposition inserted after a layer. It increases age depth without increasing deposit thickness.
- TPQ and TAQ
- Terminus post quem and terminus ante quem checks used to see whether the modeled interval is chronologically plausible.
- Calibrated range
- An external chronological interval, such as a calibrated radiocarbon range, that should overlap the modeled interval if the assumptions are compatible.