Tree-Ring Dating of The Fort- Part II
In total, 17 samples were collected for tree-ring dating of the DuBois Fort. In every case, the numbers assigned to the oak samples were the same as those inscribed on the logs for historical documentation. See Figure 1 and Table 1 for details.
The wood core samples were processed following well-established methods of dendrochronology. They were taken to our Tree-Ring Lab where they were carefully glued onto grooved mounting sticks. The wood cores were than sanded to a high polish to reveal the annual tree rings clearly. The rings were than measured to a precision of ±0.001 mm.
The actual cross-dating procedure involved the use of a computer program called COFECHA (Holmes 1983), which uses a sliding correlation method to identify probable cross-dates between tree-ring series. Experience has shown that this method of cross-dating is superior to that based on the skeleton plot method (Stokes and Smiley 1968) for oaks growing in the northeastern United States. It is also very similar to the highly successful CROS program used by Irish dendrochronologists to cross-date European oak tree-ring series (Baillie 1982),
We used COFECHA to first establish internal or relative cross-dating among the house timbers. This step is critically important because it locks in the relative positions of the timbers with each other and indicates whether or not the dates of those specimens with outer bark rings are consistent. Having done this, we compared the internally cross-dated series with independently established tree-ring chronologies from old living trees and historical tree-ring material. All of the “dating masters” used are completely independent of the samples taken from the DuBois Fort.
The results of the dendrochronological dating of the DuBois Fort oak timbers are summarized in Figure 1-Figure 2 and Figure 3, with details on the dating of each timber given in Table 1.
A. Middle and Rear Cellar Rooms
Of the eight oak joists sampled for dating, six provided cutting dates of 1703 (two in the rear room and four in the middle room; see Figure 1). This result Is extremely robust (see Figure 2). The mean chronology of these timbers has a correlation of 0.75 with the dating master developed from the Jean and Abraham Hasbrouck houses. Indeed, the two are so similar as to make it a virtual certainty that the oaks used for the construction of all three houses came from the same local forest or woodlot. The 1703 date also means that these two cellar rooms were part of the original construction of the DuBois Fort. In all likelihood, the oaks were cut in late 1703 or early 1704, with construction completed sometime in 1705 per the historical documentation.
Two joists in the rear cellar room did not cross-date with the dated joists, nor did they cross-date with any oak dating master in the Hudson Valley, northern New Jersey, and eastern Pennsylvania. The reason for this is unclear. In addition, none of the oak logs lying on top of the fireplace hearth cross-dated with any master. These logs may have come from a totally different location and at a later date.
B. Front Cellar Room
Of the five oak joists sampled for dating, three provided firm cutting dates of 1835, while one produced a cutting date of 1836 (see Figure 1). The fifth joist did not date out. The fireplace lintel produced an outer-ring date of 1832. This slightly earlier date may reflect the loss of some sapwood rings during the creation of the lintel.
Compared to the middle and rear cellar rooms, the mean chronology of the front cellar room cross-dated much more weakly with oak dating masters from New Paltz (the same used in Figure 2), northern New Jersey, and eastern Pennsylvania. Yet, all produced the same outer-most date of 1836 with a statistical significance p<0.0O1.
The best result, using the eastern Pennsylvania master, is shown in Figure 3. With a correlation r=0.47, the result visibly weaker than that shown in Figure 2, but still highly significant statistically. By the time of the construction of the new wing to the DuBois Fort, it is likely that the local supply of building timber was depleted. In addition, local commercial sawmills were probably producing dimensioned lumber for construction from a wider variety of timber sources. Consequently, the weaker cross-dating for the front cellar room is not’ that surprising. Yet it is still firm enough to say with certainty that the DuBois Fort addition was constructed sometime after 1836.
Based on a dendrochronological analysis, the original portion of the DuBois Fort was constructed from trees cut in late 1703 – early 1704. This result strongly supports the 1705 construction date that was obtained from numerals on the iron rods used to reinforce the stone walls of the house. The tree-ring chronology obtained from the DuBois Fort oak timbers agrees remarkably well with those from the Jean and Abraham Hasbrouck houses. This indicates that all three houses were built using the same oak timber source.
The addition to the DuBois Fort over the front cellar room was constructed after 1836. The quality of the cross-dating is much weaker, but still unequivocal. The fact that the tree-ring chronology from this room cross-dates equally well with a number of master chronologies in the Hudson Valley, northern New Jersey, and eastern Pennsylvania suggests that the wood used for construction was probably not local.