From the ruins of the terror attacks on the World Trade Center in 2001 sprouts an amazing link to Revolutionary War-era Philadelphia. And the key to this discovery lies with tree rings.
In 2010, excavators in New York's Lower Manhattan discovered buried deep in the ground the remains of a wooden ship and—according to a new study—that ship was built using timber that had been harvested from old-growth forests in southeastern Pennsylvania around 1773.
Independence Hall and other Revolutionary-era buildings in Philadelphia were constructed using timber harvested around the same time and from the same area, according to Edward Cook, a tree ring scientist at Columbia University in New York and the senior author of the study, which was published in the July issue of the journal Tree-Ring Research.
The ship—a type of sailboat called a sloop—was likely built in a small shipyard in Philadelphia soon after the timber was harvested. (See )
Two decades later, in the 1790s, it was deemed junk and the ship's remains were used as landfill to extend the banks of the Hudson River and create more land in the burgeoning city of New York.
How was this information gleaned from a heap of rotting wood?
"What determined the exactness of the construction date of the ship was the use of [tree ring dating]," said Cook, "which was made possible by recovering timber from the ship.
"It really is the premier scientific method for dating structures made of wood."
An archaeologist involved in the excavation discusses the historical significance of the World Trade Center ship. This video was produced in 2010 at the time of the initial discovery.
Tree Rings Tell Time
The key relates to the way trees grow: Each year they lay down a new ring of wood around their trunk. In good years—when trees grow well—the rings are wide, and in bad years the rings are narrow.
Most of the time scientists use this information to figure out how the climate has changed over time in a particular area. (See )
But it also gives archaeologists a valuable tool they can use to date wooden artifacts by cross-referencing tree-ring sequences in different samples.
In the case of the World Trade Center discovery, the ship was made of wood from hickory, spruce, pine, and oak, but Cook and his colleagues based their findings on tree rings contained in planks of white oak, which were used to build the ship's frame.
White oaks grow throughout eastern North America, but the scientists were able to narrow in on a more precise location because they already had information on tree-ring sequences in white oaks from North Carolina to Massachusetts. All that was required was a comparison between those sequences and the tree-ring sequences contained in the boat's timber.
As it turns out, "the same pattern of growth variability in the World Trade Center boat was found in timbers in southeastern Pennsylvania," Cook said. "There is no indication that timbers came from a more remote area."
The oldest ring that they identified dated back to the late 1400s, during the time of Columbus's maiden voyage to the New World. And the youngest ring—laid down in the year the timber was harvested—dated back to 1773, the same year that Bostonians held their famous Tea Party.
The later date not only tells us about when the wood was harvested, but also provides a window into the likely construction period.
"You can make an educated leap of faith that the timber was used as soon as it was cut down, and that there wasn't any seasoning involved," said Martin Bridge, an expert on dating wooden artifacts at the University of College London in the United Kingdom.
"With shipbuilding you usually use [the wood] within a year or two [of it being harvested] because it's easier to work with," so it seems like a reasonable assumption that the ship was built in 1773 or soon after, noted Bridge, who was not involved in the study.
Short Life
Cook and his colleagues were able to surmise that the ship was probably built in a small shipyard in Philadelphia because all of the timber had the same tree-ring sequence. Large shipyards would have received wood from a variety of locations, each with different growth conditions, so experts wouldn't expect their tree-ring sequences to match up as nicely.
Once built, the ship "would have been taken down the Delaware River, used for various purposes, and then taken to Manhattan where it was scrapped," Cook said. (See )
The area in Lower Manhattan where it was found was filled in in the 1790s, so—for reasons that aren't entirely clear—the ship's working life probably didn't extend beyond a decade or two.
"If ships [in the Philadelphia area] were easy to make or build because of the abundance of timber [in the late 1700s], then perhaps there wasn't a lot of time put into building a strong vessel," Cook noted. "So the life span may have been not that great."
Another possible explanation relates to a shipworm infestation found in the timber, which may have reduced the life span of the vessel.
Regardless, the ship's journey didn't end in Manhattan.
After being excavated, it was taken to the Maryland Archaeological Conservation Laboratory, where Cook and his colleagues obtained the tree ring samples.
And plans are said to be in the works to put the historic sailboat on display in a New York museum.
If that happens, museum-goers will reflect on a ship whose heritage—like its tree rings—reflects a wide cross-section of world history.
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