Archaeological chronology aims to answer the question "When did this or that event happen?". This question can usually be re-phrased as "When was this or that thing made?", where the thing under study may be anything from a bead up to the Great Wall of China.
Most dating evidence is based upon similarity: people are almost incapable of doing anything in exactly the same way for any long stretch of time, and when they try to return to an old way of doing something, they never get all the details right. Such similarities (again on all scales of evidence) are dealt with in a more or less formalised way by means of a tool kit called typology. Collect a group of similar pots / house foundations / Great Walls, note explicitly the details that unite them and separate them from their peers, and you have a type definition. Thus defined, all types have a chronological delimitation, though many may be too long-lived to be very useful, and the presence of one type of pot doesn't rule out the parallel existence of several other types.
The very birth of archaeology as a scientific discipline is reckoned from the first chronological and typological breakthrough: C.J. Thomsen's 1821 division of Scandinavian Prehistory into three Ages where cutting tools were made of different materials. First stone, then bronze, and then iron. Chronological research is still working to sub-divide the three Ages into ever finer well-defined slices.
The definition of an archaeological period takes the form of a list of types found associated with each other: pots, houses etc. How can we know in what order these periods occurred? We still largely do this by typological seriation and stratigraphy.
Seriation is a more or less formalised process where you order a collection of pots / houses / Great Walls according to similarity. You put two pots on a table, grab a third pot and decide if it should go between the two or over to either side. This is formalised as pot 1 having traits ABC, pot 2 BCD and pot 3 CDE. Then test if the series you've established is chronological, firstly by seriation of closed find associations (graves, hoards) by the same means, then by stratigraphy. Are ABC pot sherds usually in layers located on top of separate layers with CDE pot sherds? Or the other way around? Or are they usually mixed up?
So far I've spoken only about relative chronology, where we can say with confidence that the types listed for period B fill the interval between periods A and C. What about absolute chronology, that allows us to say that the period B types appeared in the AD 10s and were replaced by period C types in the AD 150s? There are many methods, most importantly radiocarbon.
Radiocarbon dating is a complicated field of research that moves forward rapidly. Briefly put it will tell you when a certain living thing died. With current technology, the accuracy is usually counted in decades. Much of the intricacies with radiocarbon have to do with the relationship between the death of that living thing and the event an archaeologist wants to date. If you find a piece of charcoal on a settlement site, you first need to think about how it ended up there. Stratigraphy is paramount: is it under a stone foundation? Or is it in a ditch that cuts across a house foundation? If you can't answer such questions, don't even submit the sample. And you need to think of intrinsic age: the heartwood of an old oak died centuries before someone cut the tree down. A wood anatomist can judge this for you. Bones have no intrinsic age, but their apparent age is skewed by the amount of seafood the creature ate in life.
Other important absolute dating techniques are historical dating (e.g. coins with dates on them or the names of rulers whose regnal dates are known), dendrochronology (the width of tree rings varies with the weather, forming a chronological bar code) and thermoluminiscence (quartz in a brick or potsherd or hearth stone accumulates radiation energy after being set to zero by strong heat).
Techniques like radiocarbon have not made typology or stratigraphy obsolete. For one thing, you need robust typological definitions to be able to generalise the radiocarbon date of a single object to an entire group of similar ones. Furthermore, fashion changes at shorter intervals than the current accuracy of a radiocarbon date. This means that you can often get a tighter date from typology than from radiocarbon. Southern Scandinavia's Migration Period lasted about 170 years and is thus about three radiocarbon dates long. But seriating a sample of female graves from Gotland, I managed to define four successive fashion phases for the same interval.
For a more thorough treatment of the subject, see Kris's long post over at About.com.
[More about archaeology, chronology; arkeologi, kronologi.]
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How can you get a date? Start by asking your wife's permission.
More seriously, I'm wondering how eating seafood alters the apparent age of bones, and whether you mean species by species (eg comparing sharks with cows), or that it varies even within a species (eg humans in a fishing village compared with those living in an inland area such as Central Australia).
It's because the carbon that cycles in marine ecosystems does not communicate instantaneously with the carbon in the atmosphere where radioactive carbon is made. This "reservoir effect" shows on an individual level. If I eat loads of fish, clip my finger nails, quit eating fish for a year, clip my nails again, and radiocarbon date both nail samples, then they will date decades or centuries apart.
Now that would be really stupid: sample dates to c.1800, but the subject turns out to be still alive. Reminds me of the time they dug up a bog body in Northern Germany and mistook her for a girl who had been missing for about 20 yrs.
codero: It's only stupid if you ignore what is known about the effects that seafood diet have on radiocarbon dating. A lot is known about the limitations of radiocarbon dating, and citing a mistake that was made (and corrected!) does not rule out its usefulness. That's like saying that you can't balance your checkbook because sometimes you make errors when subtracting or forget about some online transaction. So check your work thoroughly and learn from the mistakes; don't just throw the whole technique away.
About the body found in the bog: how were the (police?) investigators to know that a well-preserved body was centuries old instead of days? Well, through more thorough investigation, which is what happened, and the new information was taken into account. It's easy for you to co call the cops stupid, but you already knew the answer, right?
Nobody ever said that scientists knew everything. Indeed, they go on finding out new things all the time; that' show it works. That's how (most of us) got here from the stone, bronze, and iron ages.
@timberwoof: Firstly, I did actually read and understand (I hope) the bit about seafood. Secondly, the wholly hypothetical stupid/funny situation I was referring to was someone picking up Martin's seafood-rich/deprived nail clippings, wrongly assuming a different diet, wildly misdating him as long-dead, then meeting him in the streets of Stockholm. Thirdly, the people who came out looking bad from the 2000 bog body find were not the cops but scientifically-trained forensic investigators who were VERY late in remembering that ancient human remains are sometimes found in peat bogs. So there.
Thanks for the explanation! And it's interesting that you were able to define four different fashion phases in a 170-year period. Was that just the usual pace of fashion evolution, or was it because the migration caused mixing of groups with different fashions?
That's a standard pace for typological phases in female graves. The term "Migration Period" was originally lifted from Continental written history and doesn't really fit its content in Scandinavia. There's a lot of evidence of armed raids taking place here in the 5th century, but they're too small to be called invasions or migrations. A less misleading but more boring term is "South Scandy Iron Age Period 6".
How did archaeologists know what time span to put a thing in before they had access to radiocarbon dating and other methodes?
They mainly hung their chronologies onto king lists in early civilisations with writing. Oscar Montelius for instance managed to date the Scandy Bronze Age fairly accurately by following type combination chains from Scandyland down to Egypt.
Like this: a Scandy dagger is found in a grave in Germany. This grave also contains a bronze cauldron from Italy. Similar cauldrons are found on Crete, sometimes in combination with Egyptian jewellery. These jewellery types are worn by people in Egyptian grave murals with hieroglyphic writing that gives the name of the current pharaoh. If you assume that all of these objects entered the ground at roughly the same time, you can say that the Scandy dagger type is coeval with that Pharaoh.
But before the Bronze Age, they really had no idea.
This is very interesting, thank you for both the post and the answer! So when did we start having a clue about the dating of the stone age? Today you can say that this type of stone axe is from that period of years, how is that?
Objects that can't be dated in themselves often occur in closed find contexts with things that can. For instance in graves (radiocarbon date the skeleton), hoards (date the food crust on the inside of the pot containing the hoard) and refuse/sacrificial pits (date some of the charred grain in the pit).
Which is why NO dating technique will ever become totally obsolete. You have to use whatever is available, along with a healthy dose of common sense and cleverness.