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Technically this is pre-history, but maybe someone can answer this.

I was listening to a podcast about the spread of megalithic structures in Europe. A modern basis of research on this is the dating of megalithic structures via isotopical methods. A research article related to the podcast links the origin and first exploitation of this method to "Renfrew C. Before Civilisation. Penguin; London: 1973.", which I unfortunately cannot access.

The issue confusing me, that Renfew 1973 apparently exploited first is that

However, since the 1970s, the number of C14 dates of megaliths has expanded enormously.

  • (from Paulsson 2019)

Now I'm wondering: How can a megalith be dated via the C14 method? Obviously a rock cannot be dated directly via C14, as rocks are not alive and rock formation timescales and C14 decay timescales are vastly different.

So I imagine the assigning of an age to megalithic structures must work via associated burial items? What if there are no burial items to be found, like in lone-standing dolmen?

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In general, structures like megaliths are dated by their archaeological context.

In most cases, a socket will have been excavated when the megalith was erected. When archaeologists excavate that socket, we're looking to retrieve evidence which might be used to provide a date for the socket, and so for the date the megalith was erected.

For C14 dating, that evidence might be provided by fragments of charcoal buried when the socket was back-filled around the megalith, or tools used to excavate the socket ( things like antler picks etc.).

Any organic material - not just artefacts that were deliberately buried at the time - can be used to obtain a C14 date.

When we excavate a site we're looking for edges that indicate features. We can trace the edge of the socket - and any later features that have been dug into it. The socket is a 'context', the megalith is a 'context', and the fill of the socket is also a 'context'.

We use these contexts, and the observed relationships between them (e.g. context x cuts into context y, therefore context y must have been there before context x was dug, and so context y is older than context x) to construct a Harris matrix. This allows us to understand the relative chronology of the site. The Wikipedia article has some helpful diagrams that illustrate the process.

Techniques like C14 allow us to get an absolute chronology for particular features/contexts within that site.

If the socket dug for a megalith (or any other structure for that matter) is left open for any length of time, the bottom of the hole tends to have a build up of silt, which we can observe during excavation. We thus have a fairly good idea of how closely the date that the megalith was erected relates to the back-fill in the hole. If there is no appreciable accumulation of silt in the hole, then the fill must be contemporaneous with the raising of the megalith, as must any organic inclusions in that material.

In the absence of excavation and dating, megalithic structures cannot be dated with confidence, as in the recent case of the stone circle in Aberdeenshire that was initially though to be a Neolithic recumbent stone circle, but which was actually built in the 1990s. (These monuments often didn't have an excavated socket, and so can be particularly hard to date).

Colin Renfrew's 'Before Civilization : the radiocarbon revolution and prehistoric Europe' is probably available from your local library, but it is worth noting that it is also available to read online at archive.org

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  • But how is the correlation of that organic material to the non-organic megalith clear?
    – AtmosphericPrisonEscape
    Commented Apr 12, 2019 at 1:34
  • @AtmosphericPrisonEscape Because it was buried at the time the megalith was erected as part of the process of back-filling the socket.
    – sempaiscuba
    Commented Apr 12, 2019 at 1:35
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    How is that certain? That's a story that is constructed, but what's the evidence to do so? In the first place one finds a large rock, and bunch of organic stuff that one can date. But for me it is not obvious how one associates the two with each other.
    – AtmosphericPrisonEscape
    Commented Apr 12, 2019 at 1:40
  • When we excavate we're looking for edges. We can trace the edge of the socket - and any later features that have been dug into it. The socket is a context, the megalith is a context, and the fill of the socket is a context. These are all plugged in to a Harris matrix so we understand the relative chronology of the site. Techniques like C14 allow us to get an absolute chronology for particular features within that site.
    – sempaiscuba
    Commented Apr 12, 2019 at 1:44
  • Thanks, that much is clear. Maybe I'll reformulate: Why is it clear that the socket that one finds and can date is the socket for the megalith? Could it not have formed later via some natural process? What about the pictures of dolmens without excavation sites to find the socket.. are those simply not shown from the photography angle?
    – AtmosphericPrisonEscape
    Commented Apr 12, 2019 at 1:51

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