Artifact Gallery - Tree Rings

 

 

 

 

Dendrochronology (Tree-ring dating)

Dendrochronology (dendro = tree, chron = time, ology = a science), or tree-ring dating, is a scientific tool tool that allows archeologists to find out when buildings were constructed, remodeled, and sometimes, abandoned. Although tree-ring dating itself is not an artifact, the wood that is being studied by archeologists are indeed, artifacts.

Researchers at Mesa Verde National Park and the Laboratory of Tree-Ring Research at the University of Arizona have used dendrochronology since 1923. This research is a great tool to discover when an ancient community or structure within a community was built. This is especially useful for places like the Mesa Verde cliff dwellings, where some of the wood used in their construction have been preserved for hundreds of years.

Tree Rings

As you may know, most trees grow one ring each year. The width of each ring depends on the environmental conditions in which it is grown and how much precipitation it gets each year. Since the amount of precipitation changes, some rings are wide, some narrow, and some in between. Because of this, each tree’s growth rings form a pattern over time and can be compared to tree-ring patterns of other trees. It is this pattern that has allowed researches in the southwest to create a tree-ring calendar that goes back hundreds of years.

 

Charcoal rings, such as in this charred log, can also be used for tree-ring dating. By analyzing tree ring patterns, scientists can assign a date to each ring. Since the outermost ring represents the year the tree was cut, it also indicates the approximate
time it was used in construction.

The Role of Dendrochrology at the Park

At Mesa Verde, much of the wood used to construct the cliff dwellings have been sampled for tree ring dating. The most common sample is a core,** but cross sections (slices) or end cuts have also been taken. Although not all tree-ring samples produce a cutting date, many provide a date range of varying accuracy, and a few have even revealed the exact year a tree was cut.

Because of the science of dendrochronology, archeologists have learned much about the Ancestral Pueblo people and their life at Mesa Verde. For example, by using tree-ring dating along with noting architectural changes and types of pottery found at a site, they have been able to:

• create a more detailed timeline showing when many of the ancient buildings, including pithouses, pueblos, and cliff dwellings were built and inhabited.
• map out the growth of villages and how they changed over time as rooms were added or remodeled, just like many towns today.
• discover that generations of Ancestral Pueblo people built their homes or villages in the same location over time.
• learn that the Pueblo people recycled! They sometimes used the roof beams of old structures and reused them in new construction.
• provide a date the Ancestral Pueblo people moved out of area. The last tree-ring date in the region is 1281. So the last year anyone cut down a tree to construct a new building was 1281. Using this knowledge, researchers have concluded that people began to migrate by mid to late 1200s. They believe that within the next 20 years, the Ancestral Pueblo people continued to move away, and by 1300, had left the Mesa Verde and Four Corners Region.

 

Gathering a wood core sample.

Wood Coring

** a core is a pencil- or dowel-shaped piece that is drilled from within original wood that was once used in the construction of a structure, such as a roof beam, door lintel, or wall peg. The core is taken from its diameter, and usually represents a cross-section of the wood. The tree rings revealed on a core can sometimes provide a “cutting date,” meaning that it can tell researchers when that piece of wood was cut. This method of collection protects the original wood, and allows it to remain in place so there is no damage to the ancient structure.

Photo: Archeologist Nichols is coring a construction beam at Long House in the 1960s. By studying the core, researchers learned that the tree used for the beam was cut in 1275.

 

 

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• For more information on tree-ring dating at Mesa Verde National Park, visit https://www.nps.gov/articles/tree-ring-dating-at-mesa-verde.htm.