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Decoding the Past: The Work of Archaeologists
Interpreting the Evidence
Introduction - The Puzzle of the Past - Site Seeing - Digging in the Dirt - Interpreting the Evidence - Decoding the Past

During and after an excavation, an archaeologist confronts a bewildering collection of artifacts, drawings, and photographs to decipher and relate to one another. Using both relative and absolute dating methods, an archaeologist can often place a site within a larger chronological framework.

In relative dating, archaeologists interpret artifacts based on their positions within the stratigraphy (horizontal layering) of the soil. The study of stratigraphy follows the excavation axiom "last in, first out"--meaning that an archaeologist usually removes soil layers in the reverse order in which they were laid down (see Figure 1).

In relative soil dating, archaeologists follow two general principles known as terminus post quem and terminus ante quem. The first terminus post quem, refers to the notion that a datable object provides only the date on or after which the layer of soil that contains it was deposited (see Figure 2). In contrast, terminus ante quem refers to the concept that all the soil below a solid, undisturbed layer dates before that layer (see Figure 3).

Relative dating of a site's stratigraphy often depends on the absolute dating of excavated materials and artifacts. One of the most widely used methods of determining the absolute date of organic materials is radiocarbon (carbon 14) dating . Because all living organisms contain a radioactive form of carbon (carbon 14) that decays at a known and steady rate, archaeologists can determine an organic object's age (if it is less than 40,000 years old) by measuring the amount of carbon 14 remaining in the object.

Dating inorganic materials is also quite challenging, because relatively few artifacts come labeled with a date of manufacture. In fact, pottery, the most common type of artifact found at archaeological sites, seldom contains obvious indications of its age. Archaeologists sometimes use thermoluminescence dating to establish the age of pottery. This technique is similar to carbon 14 dating in that, like organic substances, pottery contains small amounts of radioactive elements that decay at known and steady rates. An archaeologist can determine the age of a pottery fragment by measuring the remaining amount of radioactive elements that it contains. Another way of dating pottery and other inorganic materials is through typology — comparing undated samples with those from associated sites that have been dated through previous excavations.

Stratigraphy Diagram

Figure 1: Simplified stratigraphy displaying the chronological progression of soil layers.

Figure 2: An example of terminus post quem. The 1885 coin in Layer E establishes that Layer E dates from on or after 1885. It follows that the pottery fragment in Layer D and the bottle cap in Layer B likely date from or after 1885 as well.

Figure 3: An example of terminus ante quem. Layer C is an undisrupted tile floor of the 1860s. It follows that Layers D, E, and F date before the 1860s.

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