Volume 25, Number 6 ● March 23, 2017
Dendroarcheological Study Confirms Date of Suspected Revolutionary War-Era Ship
In late 2015, Wetland Studies and Solutions, Inc. (WSSI) archeologists discovered remnants of one of the earliest historic buildings in Alexandria, Virginia, a circa 1755 public warehouse, during their excavations along the Potomac River waterfront. Carr City Centers had contracted with WSSI to provide archeological investigations at the Hotel Indigo site in order to comply with the city historic preservation ordinance. WSSI’s excavations also uncovered the remnants of a late-18th century ship, a bulkhead wharf, and a large wooden box privy in the sandy soil a few feet away. The wooden elements of all of the features were surprisingly well preserved, due in part to the anaerobic conditions beneath the Potomac River water table. You can read more about the Hotel Indigo site in our January 2016 Field Notes and on our website, where we have photos and a link to the NBC Nightly News coverage.
Based on documentary research and historic map evidence, we knew that the Hotel Indigo site had been filled and the remains of the early occupation were buried beneath the water table for at least 220 years; however, in order to determine the age for the buildings and other objects, our archeologists used a combination of relative and absolute dating methods. Because of the excellent wood preservation, one of the obvious methods to help us obtain a more precise date for the features was dendrochronology.
What is dendrochronology?
Dendrochronology, the science of dating the annual growth ring of trees, requires an understanding of both biology and statistics, according to Michael Worthington from the Oxford Tree-Ring Laboratory, but the premise is simple: seasonal climatic variations can be observed in the varying widths of the annual tree rings. Many species of trees, including white and red oaks (Quercus alba and Q. rubra) grow more rapidly in the spring; thus, the inner portion of their growth rings, or “early wood,” is less dense and lighter in color, while the “late wood,” which forms more slowly in the summer/early fall after the leaves have been produced, is denser and darker in color.
The tree ring samples from a site can be compared first to each and then combined, if possible, into a “site master”, which in turn can be compared to other previously dated sequences called “reference chronologies.” A combination of visual matching and computerized statistical analysis is used to indicate the validity of a cross match.
The Oxford Tree-Ring Laboratory conducted the analysis of our dendrochronological samples from the warehouse timbers, the bulkhead wharf and the ship framing. Samples were obtained by coring the timbers with a 5mm increment borer, with the exception of the wharf posts, which were cut with a saw.
Exactly how old are the wharf, the ship, and the warehouse?
All three bulkhead wharf posts matched each other and were combined into a site master. The annual growth rings from these white-oak posts spanned 136 years and, when compared to the 800 reference chronologies that have been established for the East Coast of the Unites States, matched the dates 1638 to 1773.
Samples that retain the bark and the sapwood (the growth rings just beneath the bark) can be more precisely dated. For example, if early wood growth but no late wood is found just beneath the bark, the tree was likely felled in the spring – although the actual month cannot be determined. One of the three bulkhead wharf posts retained a bark edge, which allowed assignment of a “precise” felling date for the post – and therefore the construction of this wharf – to the winter of 1773/4.
Unfortunately, the wood at the Carlyle warehouse was too soft to retain any samples; further dendrochronological analysis may be attempted after the wood has completed conservation.
The ship was constructed of white oak but clad in expendable pine boards. Mick Worthington took 15 samples from the timbers, however, only 11 provided data for the dendrochronological analysis. Three of the timbers matched each other and were combined into a 124-year site master found to span the years 1603 to 1726. The last ring sample that dated to 1726 contained 15 additional rings, which were visible but too soft to accurately measure; therefore, Worthington determined that the timber had been felled – and the ship constructed – sometime after 1741. All of the reference chronologies that the ship matched were from Massachusetts, suggesting that the ship had been constructed in New England.
What do the ages tell us?
Dendrochronological analysis helped us more precisely date the sequence of events at the Indigo Hotel Site. Soon after the establishment of Alexandria in 1749, local landowners began to cut down the high bluffs overlooking the Potomac and fill the adjacent tidal flats between West’s Point and Point Lumley in order to improve access to the river. This process was completed by 1798, according to historic maps, and enough land had been created for the laying of Union Street along the waterfront. Based on the results of the dendrochronological analysis, we know the northwestern portion of the Hotel Indigo site was infilled prior to 1773/4; and the northeastern portion shortly thereafter, as the ship remnants that were used as the framework for engineered fill were found abutting the 1773/4 bulkhead wharf. The name and owners of the ship may never be determined, but based on the dendrochronological analysis, the ship was constructed around the time that the town of Alexandria was established and possibly served its owners several decades before being dismantled and discarded in this location.