Page 140 - Encyclopedia Of World History

490 berkshire encyclopedia of world history

History cannot give us a program for the future, but it can give us a
fuller understanding of ourselves, and of our common humanity, so that we
can better face the future. • Robert Penn Warren (1905–1989)

the fifteenth century by Leonardo da Vinci. In the first Because we know the precise year each of the living tree’s
three decades of the twentieth century, University of Ari- rings correspond to, we can now fix the dead tree in time;
zona astronomer A. E. Douglass, attempting to assess a its final ring was added in the tenth year of the living
possible correlation between sunspot activity and climate tree’s existence, and we know what year that was. By
change, examined fluctuating tree ring width and is cred- repeating the same procedure numerous times with suc-
ited with recognizing the potential of tree ring analysis in cessively older trees overlapping in time, dendrochro-
archaeological dating. In the American southwest, where nologists have developed master tree ring width
tree ring dating, or dendrochronology, has been particu- sequences that reach back more than ten thousand years.
larly useful in archaeological application, the width of an When an archaeological specimen of wood is found—for
annual tree ring is proportional to the amount of rainfall example, a log beam in an ancient pueblo or a fragment
that fell in the year the ring grew; thick rings are added of log used in a prehistoric palisade—its ring width
in years when precipitation is abundant, rings of inter- sequence is examined by computer and its fit along the
mediate width are added when rainfall amounts are master sequence is determined. In that way, the actual
intermediate, and thin rings develop during years when year in which the archaeological specimen died or was
rainfall amounts are meager. cut down can be established. If we assume that the log
This yearly variation in tree ring width allows re- was used soon after it died or was cut, that year can be
searchers to extend a tree ring sequence back in time. associated with the archaeological site in which it was
Dendrochronologists accomplish this by developing found. Different master sequences have been worked out
what is called a master sequence for a region by begin- and are constantly being expanded and refined for vari-
ning with a living tree.That tree, like all trees, exhibits a ous reasons all over the world.
non-repeating, more or less random succession of thick,
medium, and thin rings reflecting the non-repeating pat- Dating with Style
tern of high, medium, and low yearly rainfall amounts New technologies are constantly replacing old ones, and
during its lifetime.This tree’s sequence of ring widths will styles come into fashion and then fall out of favor. Most
be a close match to that exhibited by all the other trees of us are familiar with the progression of technology and
of the same species growing in the region since they were the vicissitudes of fashion in our own culture. Most
all subjected to the same rainfall amounts as these varied would have no trouble guessing the approximate date of
from year to year. The living tree anchors the master an automobile with lots of chrome and sharp fins (styles
sequence in time, as the dendrochronologist knows the popular in the 1950s) or a photograph in a college cat-
precise year in which each of its rings were created sim- alogue showing male students with long hair and faded,
ply by counting backwards from the outermost, most torn jeans (the late 1960s and early 1970s).
recently grown ring, which represents the current year. Archaeologists may accurately determine an artifact’s
Next, the sequences of tree rings exhibited by a num- age in much the same way. If a unique style of making
ber of dead trees are compared with the rings of the liv- spear points or pottery has previously and consistently
ing tree in the search for a substantial, matching series of been dated to a particular point in time, then when
tree ring widths. If the lives of the living and any of the another spear point or potsherd is found that matches
dead trees overlapped in time, their ring width sequences the known object’s form and style, archaeologists pro-
will match for that period of overlap. For example, if the pose that the newly found artifact dates to a similar
succession of varying widths of the innermost ten rings period. For example, when confronted with a long, nar-
of the living tree match in size and order the outermost row spear point with a shallow channel running up less
ten ring widths of a dead tree, in all likelihood those two than half the length of each face of the point, a New
trees were both alive during that same ten-year period. World archaeologist will confidently conclude that the