Archaeology and Tree-Ring Dating
by Bekah Schields
First, let’s break it down. Dendros – having to do with trees. And chronos – having to do with time. A dendrochronologist is a professional who studies tree rings to determine dates and the chronological order of past events. Dendrochronologists create master sequences of tree ring data going back thousands of years in some locations. These sequences assist archaeologists who use them to precisely date archaeological sites that have timber used in the construction – like pueblos.
Dendrochronology was first studied in 1904 by Dr. Andrew E. Douglass, an astronomer in Arizona. As he was analyzing the climate, he noticed that the trees around him all showed the same ring pattern, as they had all experienced the same climatic conditions. Douglass began studying Ancestral Puebloan sites in the Southwest to learn more about climate further back in time.
Dendrochronology works like this, trees add rings predictably every year and in certain patterns; putting in thick rings for years with lots of rainfall and thin rings for years with less. These tree ring patterns are completely unique and never repeat themselves. By using a “bridging” method, dendrochronologists can piece together a timeline of tree rings going back hundreds of years. Let’s pretend there was a drought that happened several years ago which shows up in trees as a very narrow tree ring. For trees that just recently started growing, the tree ring that denotes the drought will be in a different location than for a tree that is very old. By piecing together these different positions of notable tree rings, dendrochronologists can create what is known as a master
sequence for a region.
In the Southwest, most pueblos were made using timber to support roofs and line doorways. Many of these sites still have wood that has been preserved, that still exhibit tree ring patterns. Archaeologists use drills to remove small cores from these beams, removing only a thin tube about the size of a straw, as to not damage these important ancient structures. By comparing the master sequence and the patterns in the cores, archaeologists can often retell the story of the development of the pueblo; which rooms were constructed first, when additions were added, and when repairs or remodels took place can all be seen through dendrochronology.
Check out our lesson about Dendrochronology in our newest Investigating Shelter curriculum: Investigating the Puzzle House Pueblo! FREE sample lesson, Part One: Geography