 | Wilson Butte CaveA NATIONAL REGISTER ARCHAEOLOGICAL SITE |
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Stratigraphy and CrossdatingObjective: In their study of stratigraphy students will use an activity sheet to:
Skills and Strategies: Knowledge, comprehension, application, analysis, evaluation, observation, discussion Materials: Five books of any size, “Site Near Richfield, Utah” and “Cross-dating” activity sheets for each student. Background: Natural material such as rocks, soil, and plant and animal remains occur on the earth’s surface and can accumulate in layers. Each layer may be distinguished by its physical characteristics: color, texture, and structure. Similarly, materials of human origin are also deposited onto the earth’s surface. In archaeological sites natural and human-generated materials occur together in layers. These layers, called strata, form a record of past events that archaeologists analyze and interpret. The materials deposited first are the oldest and are always found at the bottom of a given stratigraphic section. The most recently deposited materials are the youngest and are always at the top. This concept is known as the Law of Superposition. It always applies except when some type of disturbance has occurred. Strata in archaeological sites provide archaeologists with temporal and spatial information. All of the artifacts in a given stratum will be of approximately the same age, while those in strata above or below will be younger or older respectively. Cross-dating can indirectly establish a date for artifacts and sites. Artifacts such as stone points and pottery were made in distinctive styles through time. A modern analogy is automobiles: one would not mistake the style of a car made in the 1920s with one made in 1990. If an arrow point was found in association with a hearth that was radiocarbon dated to be 500 years old, it is assumed that the arrow point is the same age. When that style of arrow point is found at another site, the archaeologist would assign the site and the arrow point an age of approximately 500 years. Often cross-dating is the only method archaeologists have to determine the age of sites. When an archaeologist site is vandalized or artifacts are removed, knowledge about past cultures is lost forever. Damage to stratigraphy by unauthorized digging destroys the information that could be obtained under controlled scientific excavation. The removal of diagnostic artifacts from sites often removes all possibility to determine the site’s age. If you see anyone digging in an archaeological site or taking artifacts, report them to law enforcement authorities. Setting the Stage: Stack five books on a table. Tell the students that the books were placed in their positions one at a time. Ask them if they know which book was placed in position first. Which was placed last? This illustrates the Law of Superposition. Now have the students imagine how thick the dust would be on a table if no one dusted it for a year. Each book represents a layer of sediment built up in a similar fashion for hundreds or even thousands of years. Procedure: 1. Using the “Site Near Richfield, Utah” activity sheet as a guide, draw a layer near the bottom of the blackboard. Show how artifacts are deposited as people live on top of the layer. Then a new layer of sediments is deposited on top of that, by natural processes or by another group of people leaving different types of artifacts. This happens several times until the stratigraphy is built up to present day levels. 2. Distribute the “Site Near Richfield, Utah” activity sheets to the students and have them answer the questions using the information on the stratigraphy. 3. The artifacts on the “Site Near Richfield, Utah” activity sheet have been dated based on the age of the stratum in which they are found. If you found similar artifacts elsewhere, would you know approximately how old they are? Yes. This concept is known as cross-dating. An artifact type that has been dated in one place can be dated when found elsewhere. 4. Give the “Cross-dating” activity sheet to the students. Have them determine the approximate age of the artifacts based on the information from the “Site Near Richfield, Utah” activity sheet. 5. Ask the students if they would be able to study the stratigraphy of a site if the strata had already been mixed up by illegal digging. If someone took an arrow point, what kind of information would he or she have removed from the site? Site Near Richfield, Utah, Activity Sheet Answers 1. Modern Americans Cross-Dating Activity Sheet Answers 1. 10,000 to 12,000 years ago Next page: What was found at WBC?
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