Seriation orders separate graves. On a town site, archaeologists have something better: layers. Petrie noted the depth of his finds; his successors learned to read layers as a record of time.
Key idea
Soil, rubbish, collapsed walls and windblown sand pile up over time. In an undisturbed sequence, lower layers are older than the layers above them (the law of superposition). Recording which layer every find came from lets archaeologists date finds by their context, and date layers by their finds.
Why it works
A layer can only be laid down on top of something that is already there. So whatever lies beneath it formed first. A find in a layer gives a "no earlier than" date: a layer containing a seal of Ramesses II cannot have formed before his reign, although it could have formed long after. Archaeologists call that date the terminus post quem, "the limit after which".
| Layer (top first) | What it is | Finds |
|---|---|---|
| 1 | windblown sand | nothing |
| 2 | collapsed mud-brick from a house | a seal naming Ramesses II |
| 3 | house floor | pottery of the early New Kingdom |
| 4 | pit, cut down from the floor of layer 3 into layer 5 | a Roman coin |
| 5 | older town debris | pottery of the Middle Kingdom |
Worked example: Reading the section
In the section above, which is older: layer 3 or layer 2? What is the earliest that layer 2 could have formed?
- Layer 2 lies on layer 3, so layer 3 is older.
- Layer 2 contains a seal naming Ramesses II, so it formed during or after his reign, which began c. 1279 BC.
- It could have formed later than that, but not earlier.
Answer: Layer 3 is older; layer 2 formed no earlier than the reign of Ramesses II.
Worked example: A pit that breaks the rule
Layer 4 is a pit with a Roman coin, cut from the floor of layer 3 down into layer 5. It sits lower than layer 3. Is it older?
- No. A pit is dug down from a surface, so it is younger than every layer it cuts through.
- The coin shows the pit was filled in Roman times or later, more than a thousand years after the house floor.
- This is why excavators record the edges of pits, burrows and foundations: they mix the order.
Answer: No: the pit is the youngest feature in the section, apart from what lies above where it was dug.
Common mistake
Assuming the deeper layer is always older. Pits, burrows, graves, wells and building foundations can carry later things down into earlier layers. Look for the cut: the edge of a disturbance shows where the order breaks.