J Joseph Naylor with Dr. Matthew Bekker, Geography Department, BYU

Introduction

Dendrochronology is the process by which tree rings are measured and compared with others to determine the age and growth rates of trees. It has a variety of emphasis that allow for the extensive use of tree rings in studying many different fields. Some of these include; dendroclimatology, the study of climate patterns, dendromorphology, the study of natural hazard events (earthquakes, avalanches, mass movements), and dendroarcheology, the study of old wooden structures to determine cutting dates and construction practices.

Dendrochronology has been used to date structures throughout the United States with structures both prehistoric and historic in nature by using a process using tree-­‐ rings to determine the age and growth of trees. The interpretation of the samples that are collected is done by a process called “cross-­‐dating” which is critical to figuring out the age of the trees. Cross-­‐dating is the process where you take samples of unknown ages and compare them to samples that have already known ages. As someone takes samples from living trees, they know the outside ring of the tree that they have sampled because it will be the year in which it was sampled, but when you take samples from historic log structures the outside ring is unknown. That is why cross-­‐dating is so critically important to the dating of log structures.

Methodology

The beginning of the project was a pilot study that allowed the opportunity to get an idea of which timbers would be good specimens to be used in the sampling process. This we did on February 22, 2013 with permission from the property owner, Roscoe Briscoe. After arriving at the site we looked for indicators on the logs that told us that we would be sampling the true outside of the log. By doing this we could be sure that the last measurable ring that we measure is the last ring that the tree grew. Otherwise we could possibly corrupt the entire process if we assume that it is the last ring the ring grew and it in reality isn’t. Good indicators of sampling the outside ring include tree bark, beetle galleries (who burrow between the bark and wood) and patina, a shiny and smooth surface that is generally directly underneath the bark.

After we found several samples that had sufficient outer ring indicators we took samples of the logs. These samples were taken from the timbers that make up the walls of the structure and not from the timbers that made up the interior of the cabin because these had been through several fires that have charred the wood. These exterior logs were then samples from the bottom of the log where there was the evidence of outer rings. The sides of the logs had have been hewn to make them straight, so they were not a place that we took samples from.

After we found the location of where we wanted to sample, we took a marker and marked the log in the location that we wanted to drill into. This mark on the wood would be an indication once we removed the core whether or not we retained the outside of the wood that we need. Then we used a tree-­‐borer that extracted a pencil-­‐sized sample that would be used for the dating process. These holes are unnoticeable to the untrained eye, and did not affect the overall sturdiness of the structure nor did they cause any noticeable damage to the cabin. Each sample then had specific information recorded about it in a field notebook that was used to identify the samples once they were ready for examination in the laboratory. Such information that was gathered included the location on the cabin where the sample was taken, what outer ring indicators we saw on the log, whether the sample broke into pieces and whether the mark was retained on the outside of the sample. These samples were then marked and then brought to the dendrochronology laboratory located on the campus of Brigham Young University.

First the samples were mounted on a wooden mount where they would eventually be sanded. The sanding process will help expose the rings of the sample, for easy counting and measuring. Once the sanding was finished, the samples were then examined under a microscope so that the rings could be more easily seen. The rings were then counted and then measured by a program that can measure to the nearest 1/100th millimeter.

These measurements were put into a computer program where statistical tests were run on the measurements to see how well the various samples from the cabin correlate with each other.

Results

After we had finished measuring the samples and visually cross-­‐dating the data we came up with a time series plot that shows us the age of the samples. This graph shows us now only the variance in the age of some of the samples but also a variety of outer rings and cutting dates. This was made after we measured the samples and cross-­‐dated them with other chronologies of Northern Utah. After several chronologies were correlated it was found that the chronology that correlated the best was the Deseret Peak chronology. When compared using the Pearson’s R Correlation Coefficient it was found to be a high value of 0.57. This gives us a good reason to believe the cutting dates.

Conclusion

After we found the correlation between William Hawk’s Cabin and the Deseret Peak we made a graph to show the relationship between the two chronologies to compare how well they match for the years that we have in our samples. We found that the proposed date when the cabin was built 1848-­‐1852 was off by a few years. Preliminary results show that there are several samples that have a cutting date of 1855 while there are a few others that have a cutting date of 1852. This leads us to believe that the cabin was built in 1855 and was built with lumber from previous structures, a common building technique in desert climates.