Why did the people abandon this cave village?
| What WAS the Weather Like? |
AN INTRODUCTION
The Study of Tree Rings
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Why did the people abandon this cave village? |
Archeology: DENDROCHRONOLOGY
AN INTRODUCTION |
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Dendrochronology, the study of tree rings, is a valuable tool in both archeology and in the history of weather. What is so great about it is that it has a precision of about one year while most other dating techniques have errors of 50 years or more. Only the study of bands within the frozen icecaps of Greenland and Antarctica can compare with tree ring analyses.
Taking a closer look, you will see that it is obvious that the rings are not evenly spaced. Some are farther apart than others. If it was growing in a good year - lots of sunlight and water, the tree was able to produce more wood than in years of draught or if the sunlight was diminished by a lot of smoke in the air such as from a volcanic eruption somewhere upwind. Thus, you see, the trees stores a record - a history - of the weather it has experienced over its lifetime.
Of course it is not a very ethical idea to immediately rush out to the nearest forest and start cutting all the trees down just so that you inspect their growth rings. Dendrochronologists are kinder to the environment than that! What they do is drill out a thin core of wood, and they try to keep the bark attached so that they know where the most recent wood is. (If you want to know what a coring bit looks like, ask your local hardware store people to show you one. They will probably show you a short one, because it would be used to make pegs for furniture construction. The forester would have a much, much longer one.
And of course the scientist doesn't need to drill all the way through the tree to get a core, because the one side should pretty much reflect the same banding patterns as the other. As a matter of fact, one doesn't really need to even drill all the way to the center of the tree. Once the core is extracted from the corer, it is carefully sanded and then varnished to make its rings and other structures such as pores and cells (microscopy, too!) stand out.
Of course, if you attack a forest with your corer, you will get only rings that extend backwards from now for as long as that tree has been growing. That may not be very long - perhaps only 150 years for some of the older trees in your woods. What you want are cores taken from trees today, 20 years ago, 75 years ago, 110 years ago, and earlier. However, all the current trees are long rotted away or have been cut up to be used as lumber somewhere.
Hey, did I hear someone mention old construction lumber? Great idea! Take, for example, the cliff dwelling in Mesa Verde National Park (shown next to the title on this page). In some of those old buildings (archeology, remember!) are logs that are used as floor and roof joists. What if the Park Service allowed us to take a core of one of those? Maybe it would help us to determine when that tree was cut down, and thus when the structure was built! But we notice that the banding doesn't match anything in living tree we cored out back in own yard. This could be for two reasons: (1) the Mesa Verde log was much older, and (2) our back woods is not near where the Mesa Verde log grew and thus different weather conditions were encountered.
So we need to use trees and logs that all grew in pretty much the same area. AND it would be good always to use the same species of tree as some trees make bigger rings than others. Walnut trees make tighter rings than do pine trees, for example. So, for learning purposes, hereonafter, assume that we are speaking about the same species of tree.
So we went to Mesa Verde National Park and looked around for places where the ancient people might have found trees growing: on a nearby plateau, perhaps! After we cored a few modern trees, we scrounged around for logs of long dead trees. We found a more recent ruin that had a few good logs. And we found a few logs in other caves - some washed in during flash floods. We cored all of these, and then lined them up so that their banding patterns matched. (This is sort of like what the FBI does when comparing bullets to see if they had been fired from the same gun.)
Once they are all lined up, we make a composite sketch. Can you find the core from the Mesa Verde log? I would ask you to try to figure out when the tree was cut down, but this is only a hypothetical bunch of cores, and they don't have enough rings.
Something to do: If you want some practice, click on the picture immediately above, and it will come up alone on a separate page. Print it, and then cut out the various parts and see if you can line them up. At first, don't use the composite sketch - pretend you haven't made it yet!
The next bit of archeological detective work returns you to the question of why these people left. If it was due to a weather phenomenon, it should be recorded in the rings that soon follow the building of the dwellings. If there was an extended number of years of drought, then we should see a tight bunch of tree rings shortly after the Mesa Verde log was cut down and used for building. In the above example, we do see this, and so we must conclude that the site was abandoned for drought reasons such as crop failure. Of course, this could be complexed with an epidemic that killed the people; or perhaps there was an invasion by another group of people. Afterall, we do know from sagas, that the Navajo were moving south out of Canada at that time. Thus the Navajo might have displaced these people, who, in turn, moved yet further south. Afterall, take a look at the construction style in the photograph above. Doesn't this remind you of the pueblo builders, and weren't the people pueblo builders, who wandered south in the deserts of Mexico at that time to establish the Aztec empire. Cortez certainly saw the architectural styles of pueblo construction in area he conquered.
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