Archeological Dating: Retracing Time (1976)

[Music] silent it is the work of archaeologists find these remains dig into them and end the silence archaeologists reconstruct these cultures to determine how ancient people lived and one vital question they try to answer about these societies is how old old are [Music] they much of this work is being done in the southwestern United States where there are thousands of prehistoric ruins today archaeologists dig into these ruins to find the evidence of ancient life section by section layer by layer the past is revealed and interpreted at the salon ruins in Northwestern New Mexico a large team of archaeologists and other scientists is Excavating an extensive Indian peo the rooms of the ancient town are now deep pits from which rubble and Earth are carefully removed for further study the project is directed by Dr Cynthia irn Williams of Eastern New Mexico University the site has yielded thousands of pieces of broken pottery each day Dr irn Williams examines the material that has been collected as this ceremonial chamber or Koo was excavated evidence was uncovered that it had been destroyed by a great fire a stone image or fetish was left lying on the floor when the KA burned this is a very important artifact because valuable objects like this were rarely left in the places where they were used the endless digging goes on under the hot New Mexico Sun every shovel full of dirt is carefully screened to remove all the artifact effects that will help reconstruct a picture of life in the pebo pieces of pottery and a stone knife give Clues to the pbo's age discarded bones may tell what the people ate or what kind of tools they used a room full of pottery is an archaeological Treasure Trove here artifact are found just as they were left when the great fire occurred centuries ago much of the pottery is in pieces called pochards even so their placement gives Clues as to how the room was used a storage Bowl holds corn kernels charred by the fire each pochard is collected after its location is noted sometimes only a few pieces are found other times all the pieces are found and the puzzle can be put [Applause] together pot shards and small ceramic objects help archaeologists determine what kinds of pottery the inhabitants manufactured and what kinds were traded from other areas each pochard is marked the code indicates the site where in the site the piece was found and the number of the piece the data recorded on the pot shirts is entered on computer Punch Cards computers have revolutionized archaeology because they store and process huge amounts of information a computer can be used to find and match individual pochards and thus guide the accurate reconstruction of pottery while the site is being excavated archaeologists begin to determine its age the pochards provide clues sorted by types they can be used in relative dating finding out how old the site is in relation to other similar sites all these distinct Pottery types were manufactured at places in the south West which have been previously dated at various times people from these sites traded Pottery to the people who lived at Salman it's important to know where each type of pottery has been found in the ruins to do this archaeologists use stratigraphy they go back in time by analyzing the Contex of each of the strata or layers on top is dirt that has blown in through the centuries and covered the site as we go down we go back in time the first and most recent layer of occupation in this room is a refug dump the second layer is the floor of a corn grinding room the bottom and earliest layer is the floor of a living room stratigraphy shows us that this room was occupied or used several times in the corn grinding room is a grinding stone and Mesa ver type Pottery this is the same kind of pottery that was found in the Mesa verie ruins in Southwestern Colorado this type of pottery has been dated at Mesa to the late 13th century so we know that this room was in use at that time from The Refuge dump to the Mesa verie floor and down to the lowest level we travel back in time here choco style Pottery has been found it is the same kind of pottery found at the choco Canyon site 80 kilm South of Selman at chako Canyon this type of pottery has been dated to the early 12th century so we know that this level of the room at the site was occupied about that time while pottery typology and stratigraphy are useful in relative dating several other techniques are needed for exact dating in a storage room a rare find has been made an intact wooden roof dry desert conditions have preserved the centuries old roof beams sections are sawed off and shipped to a laboratory to be dated the process is called dendr chronology tree ring dating and is done at the University of Arizona's laboratory of tree ring dating in Tucson dendro chronology depends on the fact that trees have annual growth rings these Rings vary in width from year to year depending on the amount of rainfall and heat during the growing season a number of samples have been taken from the salmon site each will be carefully analyzed under a microscope the dark area at the center of the section is the pith formed in the tree's first year each concentric ring represents one year of growth and indicates climatic conditions in that year in dry and cold years the Rings are narrow and closely spaced in years of abundant rain rings are thick and widely spaced all trees in the region show this characteristic ring pattern a chart is prepared for each sample the lines represent unusual or significant rings short lines are drawn for wide Rings long lines for narrow Rings dendro chronology has turned out to be a highly accurate method of dating ruins in the southwestern United States a master chart of tree rings has been developed going back to the year 59 BC for this area the chart for one of the logs from the salon ruins is moved along Master chart until the two match now the laboratory technician can pinpoint the date when the log was cut 1061 is an unusually early tree ring date for the Salman ruins now other dating methods will be used to cross check the tree ring dates in the Kaa samples of an ancient clay Hearth are being prepared for archeomagnetic dating clay contains magnetic materials when a clay object such as a hearth is heated and then the minerals in it will assume and preserve the direction of the Earth's magnetic field at that time samples consist of several small pillars of burnt clay taken from the Hearth molding plastic and molds are placed around the pillars Dental plaster is poured into the molds in a few minutes it hardens keeping the sample intact a compass is used to orient the sample in relation to the Magnetic North Pole the geographic North Pole of the earth is not the same as the magnetic pole the magnetic pole has wandered around the geographic pole through the centuries the dat for each position of the magnetic pole have been obtained by crosschecking with the dates on the master tree ring chart from the southwest the plaster enclosed specimens from The Hearth are sent to the archeomagnetic dating laboratory at the University of Oklahoma each sample is placed in a specially built magnetometer the sample is spun at a high rate of rotation and enclosed within electrical coils the coils detect the weak magnetic field in the sample the direction and intensity of the magnetic field in each sample is determined this data reveals the direction of the Earth's magnetic field when the Hearth was last heated and cooled when that direction is plotted on the Polar map the date is revealed The Hearth was last used in 1095 about 35 years later than the tree ring date obtained from the roof beam stone tools can be used for both relative and exact dating in the southwest projectile points that vary in size and shape depending on their age have been dated from about 10,000 BC but at the Salmon site only the most recent types have been found for exact dating obsidian tools are used the procedure obsidian hydration dating is based on the fact that from the time the tool was made it has been absorbing water through a microscope a thin hydration layer formed by the water can be observed the width of the hydration layer measured in microns tells how old the sample is calculations show that the hydration layer has been forming for approximately 880 years this Arro Head was made in about 1095 now preparations are made for yet another exact dating procedure a burned log will provide samples for the carbon4 test several logs are sent to the Smithsonian institution in Washington for Carbon 14 dating at the laboratory a sample is burned in an atmosphere of pure oxygen the carbon dioxide gas produced is collected in steel flasks liquid nitrogen poured on the flasks freezes the gas changing it into a liquid burning continues until all of the s sample has been consumed after impurities are removed the sample is stored as a liquid in this cylinder the carbon 14 in the sample is radioactive and its radioactivity can be measured the older a sample is the less carbon 14 it contains If the percentage of carbon 14 remaining in a specimen is measured its age can be determined a sample is placed in a heavily shielded radiation counter the radioactivity of the sample is counted at least twice for 1,000 minutes at a time the readings of carbon 14 activity are computed the date for this sample is 1,00 plus or minus 70 years now Dr Irwin Williams takes all of this information on dating and spends many days analyzing it from different locations in the peo she has obtained dates using a variety of methods these dates document the building occupation and destruction of the peblo and provide a starting point for understanding the people that lived here over 800 years ago the work at the Salmon ruins goes on it is a large peblo and will continue to yield information unfortunately much valuable information at other ruins on our continent is being destroyed by treasure Hunters who tear sites apart just to get intact pottery that they can sell they don't care about pots and burnt corn and bones and the location of these items all of the material that is so vitally important to the dating methods we've seen the southwestern United States is a great Treasure House of prehistoric cultures much has been lost but much is being saved by government legislation and the action of concerned citizens through efforts such as these our cultural heritage will be preserved to enrich the lives of future Generations [Music]