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8.2: Activity 1 - Stratigraphic Dating and the Harris Matrix

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    74779
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    Jess Whalen, Mt. San Jacinto College

    Part 1. Stratigraphic Dating: A Café Scene

    Relative dating methods establish the date of something as older or younger than something else rather than anchoring its age to an absolute, scaled timeline as in absolute dating. So, we determine the sequence of at least two things (two events, two deposits, etc.) and establish what happened first, what happened next, and so on.

    In archaeology, relative dating relies on stratigraphy—what material is located above or below something else. The Law of Superposition tells us that material positioned underneath something else is usually older and material overlying a deposit is younger than the deposit unless the layers have been disturbed.

    The following photos(Figure \(\PageIndex{1}\) to Figure \(\PageIndex{3}\)) depict a collection of items on a table: a cup, saucer, newspaper, and tickets. Imagine that these items are part of a single context such as a layer in an excavation and you want to determine whether they were deposited all at once as a single event or one after the other over a longer period. Examine the photos and answer the following questions.

    Behaviorism_1.gif
    Figure \(\PageIndex{1}\)
    Behaviorism_1.gif
    Figure \(\PageIndex{2}\)
    Behaviorism_1.gif
    Figure \(\PageIndex{3}\)
    1. Using the Law of Superposition, which material is the oldest?

     

    1. Which material is the youngest?

     

    1. How do you know this?

     

     

    Part 2. The Harris Matrix

    To map the succession of layers in a context (site), archaeologists use a Harris Matrix. It allows us to draw a diagram of the materials above and below other materials so we can understand the succession of deposits and determine the site’s approximate date.

    The Harris Matrix uses boxes and lines to clarify the stratigraphic relationship of the objects. Each item is represented by an individual box, and the boxes are drawn alongside, above, and below each other and connected by straight and parallel lines to show the stratigraphic relationships and, thus, their relative positions.

    Three rules are important when drawing a Harris Matrix:

    1. Draw the boxes representing all of the materials from a single layer along a horizontal plane (in the same horizontal layer). Each horizontal plane/layer must be clearly distinguishable from the layers above and below it.
    2. Draw only straight vertical and horizontal lines—no curving lines.
    3. Connect the boxes representing materials that are directly associated—are touching each other—with horizontal lines. Do not connect boxes representing materials in the same horizontal plane that are not touching other materials with horizontal lines.
    4. Connect a box to boxes directly above and below it with vertical lines. Also use vertical lines to connect boxes representing a displaced (pushed aside) item and the box representing the item that disturbed it.

    The following Harris Matrix shows boxes for five items listed as 10, 11, 12, 13, and 14. From the matrix, we can see that items 10, 11, and 12 were found in a single horizontal layer and that item 11 was touching items 10 and 12. Item 11 was found directly above item 13, and item 13 was found directly above item 14.

    Use this Harris Matrix (Figure \(\PageIndex{4}\)) to answer the following questions.

    Behaviorism_1.gif
    Figure \(\PageIndex{4}\): Harris Matrix
    1. Which item is the oldest material in this context?
    2. Which item is the youngest material in this context?

     

    1. Draw a Harris Matrix representing the collection of items in the café photographs. Use boxes to represent the items (the cup, saucer, tickets, and newspaper) and place the boxes alongside, above, and below each other based on their positions in the photograph. Connect items that are directly associated (touching) with straight vertical and horizontal lines. Be prepared to share your matrix and answers with the class.

     

    Part 3. Date Before Which and Date After Which

    Even when using relative dating methods, we are interested in establishing at least approximate dates for our deposit. We do this by establishing the terminus ante quem (the “date before which” or DBW) and terminus post quem (the “date after which” or DAW) for deposit at the site.

    Behaviorism_1.gif
    Figure \(\PageIndex{5}\)

    Terminus Post Quem – Date After Which

    The DAW is the earliest possible date for the materials. They cannot have been deposited before this date. The DAW is found by determining the latest possible use of the materials.

     

    Consider a shaving kit found in a garbage pit that contains a razor, scissors, and tweezers. You determine that the razor was manufactured from 2009 through 2012 while the scissors and tweezers were manufactured only in 2015. Thus, the earliest possible date of deposit for this collection is 2015 since some of the materials did not exist prior to that year.

     

    Terminus Ante Quem – Date Before Which

    The DBW is the latest possible date for the materials. They cannot have been deposited after this date. In this case, dates of manufacture do not work since many utilitarian objects such as the razor and scissors in our example are used for many years and even across generations. Instead, we establish the date of the earliest known event that occurred after the materials were deposited.

     

    Returning to our shaving kit, we know it was deposited no earlier than 2015 (the date of manufacture of the tweezers and scissors) but we do not know when the kit was thrown into the garbage pit. The first event we know of after 2014 is a volcanic eruption that covered the pit area with ash in August 2017. Therefore, we know that the latest possible date for deposition of the shaving kit is August 2017.

     

     

    1. Fill in the blanks in the following passage.
    In a single site context, we have two coins found inside a pair of jeans. One coin is dated to November 1998 and the other is dated to June 1992. The jeans were manufactured between July 2001 and June 2005. Therefore, earliest possible date for this context is __________. The material cannot have been deposited earlier than __________. This is the date ________ which, also called the terminus ________ quem.

     

    2. Is it possible for a pair of jeans to be worn after the last date of their manufacture? (Circle your answer)

    Yes No

     

     

    3. How can you establish a terminus ante quem for the jeans using information about the coins inside? Use your imagination!

     

     

     

    4. Determine the earliest possible date for the café scene in the photographs by examining the photographs for dates on items such as the tickets and newspaper.

     

     

    On the Harris Matrix you previously made for the café items, insert the dates you find for each ticket and the newspaper.

     

     

    5. What is the terminus post quem for the café scene?

     

     

    How do you know?

     

     

    6. A terminus post quem is also called the “________________________”. It is the _________ possible date for a context.

     

    7. How could you establish a terminus ante quem for the café scene? Be prepared to share your ideas with the class.

     

     

     

     

    8. How long is the period during which you think the café items were deposited? Why?

     

     

     

     

     

    9. What do you think happened at the café site and why? Reconstruct the sequence of activities.