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Stratigraphic Information

Here I wish to use the term stratigraphic information in the broadest sense as referring to anything which defines the relative ages of different samples or objects. Clearly in many situations this will be simple archaeological or geological stratigraphy but in other situations other information might be treated in exactly the same way.

As a trivial example, a sample taken from between two layers securely dated to 1066 and 1087 will have exactly the same chronological constraints as a sample which is simply known to have come from the reign on William the Conqueror. Both cases can be treated as a sequence of: one securely dated event 1066; the item in question (perhaps with a radiocarbon date of 950BP+-30) and finally another securely dated event 1087. In terms of a stratigraphic diagram we might draw this as:

Sequence
{
 C_Date 1066;
 R_Date 950 30;
 C_Date 1087;
};
The implications of such a simple sequence are fairly obvious in that the original probability distribution for the radiocarbon date will simply be truncated at the two dates 1066 and 1087. The value of analysis only becomes significant in more complicated situations where the implications of the stratigraphic information are not so obvious.

A very important point must be made which is that radiocarbon often do not directly date the context itself and so a naive use of stratigraphic information to refine the dating of the objects can be quite wrong. As an example sample A in pit 1 may be older than sample B in pit 2 even if pit 2 is older than pit 1.

The taphonomy of a site must be carefully considered in constructing a chronological stratigraphy from the physical stratigraphy.

In general the relative order of all samples is rarely known but various stratigraphic groupings can be defined.

See also [Program Operation] [Mathematical Methods]

Phases

A phase can be defined as a group of items for which one has no information about the relative ages but all of which share some relationships with items outside. You might for example have two radiocarbon dates:
Phase
{
 R_Date 2700 30;
 R_Date 2800 35;
};
This might then be part of a sequence.

If the samples form a coherent group then they should be enclosed within Boundaries.

See also [Program Operation] [Warning] [Example]


Sequences

A sequence is here defined as a group of events or phases which are known to follow one after another with no possibility of overlap. For example a fragment of a model might include:
Sequence
{
 R_Date "A" 2760 35;
 Phase
 {
  R_Date "B" 2700 30;
  R_Date "C" 2800 35;
 };
 R_Date "D" 2660 35;
};
The stratigraphic information from most sites can in fact be written solely in terms of nestings of phases and sequences. However, to use sequences properly an understanding of Boundaries is needed (see Steier and Rom 2000 and comments on that paper). In this program it is also possible to define a minimum gap between two events in a sequence so that you might have:
Sequence
{
 C_Date 1066;
 Gap 10;
 R_Date 950 30;
 C_Date 1087;
};
(Please note that in this manual and for this program sequences are always written in the order old to young although they can be displayed in reverse order for consistency with physical archaeological stratigraphy).

See also [Program Operation] [Example] [Warning]


Boundaries

The basic assumption underlying the analysis performed by this program is that the dates of the events being analysed are randomly selected from a uniform distribution. Without any other information, the program will assume a priori that the period from which they are selected has no limits. In practice, this is usually not the case (for associated dangers see Steier and Rom 2000 and comments on that paper) and the events are selected from a slice of time with a start and a finish. To tell the program the Boundary command is used.

As an example in the case given above we might have:

Sequence
{
 Boundary Start;
 Sequence
 {
  R_Date 800 35;
  Phase
  {
   R_Date 750 30;
   R_Date 800 35;
  };
  R_Date 660 35;
 };
 Boundary End;
};
Any coherrent group of events should be contained within boundaries in this way in order to signal that they all belong to one period.

See also [`Using boundaries]


Sequences with known age gaps

A specific type of stratigraphic information `Wiggle Matching' are sequences where the gap between specific events is known precisely in terms of calendar years. Tree rings are the most obvious example of this but some forms of sedimentary deposit also lend themselves to this type of treatment. One might draw a stratigraphic diagram for such a system as:
D_Sequence
{
 R_Date 2760 35;
 Gap 30;
 R_Date 2910 30;
 Gap 30;
 R_Date 2870 35;
};
See also [Program Operation] [Example] [Warning]

Sequences with approximate gaps

Another type of stratigraphic information which is occasionally encountered is a sequence of layers or events with a gap which is known approximately (from sedimentation rates, peat growth etc.). This is in principle very similar to the previous type of sequence:
V_Sequence
{
 R_Date 3000 35;
 Gap 30 20;
 R_Date 2910 30;
 Gap 30 20;
 R_Date 2870 35;
};
See also [Program Operation] [Example] [Warning]

Termini

To return to more common archaeological situations we must also consider events which define a terminus ante quem (TAQ) or terminus post quem (TPQ) within a sequence. If for example a coin dated to 1066 is found between two archaeological samples in a sequence it follows that the later sample in the sequence must have been (deposited) after 1066 but the earlier sample might be before or after:
Sequence
{
 R_Date 980 35;
 TPQ
 {
  C_Date 1066;
 };
 R_Date 930 30;
};
See also [Program Operation] [Example] [Warning]

Cross Linking

Occasionally there is some archaeological information which links two fairly independent sequences. The ability to deal with such relationships is provided in this program by allowing references to items in previously defined sequences. An example might be:
Sequence
{
 R_Date "A" 900 30;
 R_Date "B" 830 35;
};
Sequence
{
 R_Date "C" 940 35;
 TPQ
 {
  XReference "A";
 };
 R_Date "D" 890 70;
};
Such use of references should be used with caution and combinations of sequences and phases used where possible.

See also [Program Operation] [Warning]


Warning

A very important point must be made which is that radiocarbon often do not directly date the context itself and so a naive use of stratigraphic information to refine the dating of the objects can be quite wrong. As an example sample A in pit 1 may be older than sample B in pit 2 even if pit 2 is older than pit 1.

The taphonomy of a site must be carefully considered in constructing a chronological stratigraphy from the physical stratigraphy.