Crossdating - The Basic Principle of Dendrochronology

Crossdating - The Basic Principle of Dendrochronology

Crossdating is the most basic principle of dendrochronology. Crossdating is a technique that ensures each individual tree ring is assigned its exact year of formation. This is accomplished by matching patterns of wide and narrow rings between cores from the same tree, and between trees from different locations.

In the example illustrated to the right, we will attempt to date the construction of the Puebloan ruin (C) on the left. First, increment cores are extracted from living trees (A) to develop a living tree chonology for the nearby area. Then, the living tree chronology was extended by obtaining cores from dead standing trees nearby (B). Finally, cores taken from beams inside the ruin were crossdated with tree-ring patterns from the other two sites. This method illustrates the concept of chronology building so often used in dendrochronology.

As you can see, crossdating was accomplished by first observing an easily recognizable pattern in the tree-ring sequence of the living trees. For example, perhaps a ten-year period may have the pattern N-W-W-N-N-W-N-W-N, where "N" would indicate a "narrow" ring and "W" would indicate a "wide" ring. If a similar pattern could be found on the samples taken from old dead trees, then the samples would be considered crossdated. Keep in mind, however, that crossdating uses practically all the rings held in common between the overlapping samples to ensure that crossdating has been accomplished.

So, let's try to illustrate this basic principle with a "real-world" example. Here we have two tree-ring sequences from beams found at an archeological site in northern Arizona called Betatakin. Both cores are from Douglas-fir trees that were cut down by the Anasazi (Native Americans who lived in the Southwest about 2000 to 700 years ago) who built these ruins. The upper core was extracted from one beam, while the lower core was extracted from another beam in another portion of the ruin.

You can clearly see how these two cores crossdate against each other. Notice how the narrower rings (those pointed out with lines between the samples) are common between the two different trees. If the upper core had already been crossdated against a master tree-ring dating chronology, then we could now easily assign calendar dates to the lower core as well.

So, what causes the tree ring sequences to have characteristic patterns of wide and narrow rings? After all, if the rings were all the same width, then we could not establish any crossdating and dendrochronology would not be very hepful. This point emphasizes that, for dendrochronology to work, there must be some variability in the tree-ring sequences - in other words, there must be a full range of rings from very narrow to very wide for us to see common patterns.

This illustration on the right demonstrates the principle of sensitivity. Trees useful for tree-ring dating will be growing in environments that produce a "sensitive" series of tree-ring growth patterns. The tree to the left, however, is growing in an area where moisture is not limiting, and therefore will produce a "complacent" series of ring growth patterns. Little or no record of climate variations exist in these types of trees.

For more information about other principles used in dendrochronology, consult the "Principles of Dendrochronology" Home Page developed by Henri D. Grissino-Mayer.