When archaeologists or human paleontologists are working on any kind of site (a location where human physical and/or cultural remains have been found), there are several questions that need to be answered.
When archaeologists look at a site, they tend to distinguish three kinds of cultural evidence. First are artifacts: anything made or modified by humans that is somewhat portable. Tools, flaking debris, hammerstones, pottery, figurines, and jewelry are but a few examples of artifacts. To relate artifacts to the critical concepts of context and provenience, consider this very hypothetical example. A black granite stone whose chemical analysis indicates it could only have come from Egypt is found in situ at a site in southern Mexico dated at approximately 200 BC (BCE). In addition, the stone is covered with Egyptian hieroglyphics which indicate the origin of the stone was in Memphis, a capital of the ancient Egyptian kingdom. Assuming that indeed this stone is in situ, undisturbed in the context of an early Mexican site, then the presence of this artifact is absolute evidence that there was at least contact between Egypt and Mexico at this date, and would support the hypothesis that ancient Egyptians were responsible for the pyramids that are found in Mexico. Let me point out however that to date, there is no such evidence!
In addition, archaeological sites almost always include features: evidence of human activity that is not portable. Features can include fireplaces, postholes, walls, remains of buildings, rock art, burials and burial tombs, among other things. Features can also be the exact way tools, flaking debris, and hammerstones are scattered over an area,indicating a tool manufacturing area. Once excavated, such a feature is destroyed, so everything about it must be recorded and photographed.
Finally there are ecofacts: anything which may indicate something about the climate and general ecology of the area in which people lived. Ecofacts can include the plant and animal remains found at the site, whether brought in by humans or found living there naturally, such as small snails. The plant pollen found at the site is an important ecofact, since plants are usually more sensitive indicators of climate than animals, and the site will contain windblown fossil pollen from the time period the site was occupied. Pollen can also be found within burials, and sometimes in dried coprolites (feces), or if human bodies are somewhat mummified, in the intestinal tract.
To record the relationship of all artifacts, ecofacts, features, and human remains at a site (in other words, to record the context), archaeologists have to know exactly where everything is in terms of both vertical and horizontal space. Careful mapping and recording of everything is necessary. To do this, archaeologists tend to grid a site (one meter squares are the most common) to record both horizontal and vertical location of everything recovered. Excavation is often done in 10 cm. levels. Soil is normally screened for the smallest artifact or ecofact, and then flotation is commonly used to recover minute pieces of bone, shell, plant remains, and ultimately pollen. In this way archaeologists try to learn everything possible about the culture and its adaptation.
Archaeologists Working at an Early Man Site in Boxholm, England
The stratigraphy of the site, the natural layering, must also be recorded. The stratigraphy itself is often a feature, and always an ecofact, since it provides evidence of climate, climate change, and other environmental factors. If artifacts, ecofacts, and features are in fact from the same people or time, they are found within the same stratigraphic level. Each layer in the photo below provides information on the geology and climate of this particular location. Using the law of superposition, the oldest layer shown is the one at the bottom.
Stratigraphic Sequence from the Site of Boxholm, England
There are two general types of dating techniques, relative dating and chronometric (sometimes called absolute) dating.
Relative Dating "dates" two objects or strata or fossils in terms of each other; i.e. one is younger or older than the other. The basis for most relative dating techniques is stratigraphy, or specifically the Law of Superposition. The Law of Superposition essentially states that in a sequence of geological deposits or strata, such as seen in the picture above, the layer on the bottom was deposited first, and is hence older, then the layer above it, which was deposited more recently. Check the required site, listed below, for more information.
Fluorine Dating is another relative dating technique which can be used on bone. As bone lies in the ground it absorbs minerals. One mineral absorbed is fluorine, commonly found in groundwater. If two bones found together have been in the ground the same amount of time, the will have the same amount of fluorine present. This method is most commonly used to prove that two bones found in the same strata have indeed both been in the ground the same amount of time, i.e., the strata has not been disturbed by any natural and or human activity. The most famous use of fluorine dating has been to unmask Piltdown Man. Piltdown Man, a famous fraud in paleoanthropology, won't be discussed in this class, but if you are interested check http://www.talkorigins.org/faqs/piltdown.html(optional site).
Chronometric (absolute) dating methods provide dates in terms of years ago (y.a.), or years before present (BP). There are three chronometric dating methods you need to know: dendrochronology, radiocarbon, and potassium argon. For more detailed information on these three absolute dating methods, as well as on superposition, stratigraphy, and cross dating, you need to read the required site on dating. You should also look at the very brief videos within the site, for which you will need to have Quicktime installed on your computer. (Free Download) The required site is at http://archserve.id.ucsb.edu/courses/anth/fagan/anth3/Courseware/Chronology/02_Introduction.html Use the pulldown menu on the right side, near the bottom, to find the information on each dating method.
At the bottom of the site is a pull down menu; read about the relative dating techniques: superposition, stratigraphy, cross dating. You should also read about the absolute dating techniques dendrochronology, radiocarbon, and potassium argon. The lessons on paleomagnetism/archaeomagnetism, obsidian hydration and thermoluminesence are optional, but highly recommended to give you a more complete picture of available dating techniques. In practice of course, an archaeologist would use as many dating techniques as possible, in order to cross-check each date and method against other methods. Radiocarbon dating and potassium argon (or radiopotassium) dating are also discussed briefly in the text.