Archaeologists remove a lot of dirt when they excavate. That dirt gets scooped into buckets and then dumped into screens to be sifted so that we can look for artifacts that we did not notice in situ (in place).
The screens are shaken, and the dirt drops through leaving artifacts and rocks behind. We check the rocks to see if any are cultural, meaning they were used by people in the past as perhaps tools, or in hearths and fire rings.
We discard anything that is not cultural (rocks, twigs, leaves, etc.). Then we pick out all the artifacts, sort them, and record them on paper or digital forms. Often, archaeologists put artifacts into bags to go to the lab for cleaning and analysis, but sometimes a ‘catch-and-release’ method is used, where artifacts are placed back in the unit where they were found and covered back up with dirt.
As archaeologists excavate they pay careful attention to the stratigraphic layers. Stratigraphy is the study of layers. Geologists use stratigraphy to study layers of rock, and archaeologists use it to study layers, or strata, of soil. Soil layers are deposited through natural earth processes, and can contain human, what we call cultural, elements. Over time, sediments are laid on top of each other (by processes of decomposition, wind, water, erosion, etc.) making layers older the deeper you dig into the ground. The newest layer is at the ground surface that you can see without digging. The order of the layers is called the law of superposition.
The layers of soil give us very important information about what was happening at a site, and we take time to record several things. We determine each layer’s soil color using a special color guide called a Munsell book. We figure out the composition of the dirt–how much clay, sand, and loam are in it. We also calculate the percentage of rocks, or gravels, in each level. All of this information gives us clues about the many factors that go into how an archaeological site was formed, including whether deposits are disturbed or undisturbed.
Undisturbed deposits are those in which soil layers are in the exact order they were laid down and any artifacts in the layers are exactly in the spot they were left by the people who used them. In undisturbed deposits we can figure out time periods of layers and artifacts relative to each other. The context of the site can also sometimes help us determine how an artifact came to be where it is in the archaeological record, whether it was through loss, abandonment, caching, or discard.
Disturbed deposits are those where the natural order of the strata has been changed. Disturbed deposits often get that way from people digging into them and messing up the natural order of the layers. As one example, we sometimes find pits dug into the earth. In these pits, the dirt color and gravels percentage will be different from the undisturbed layers, and there will sometimes be a jumble of artifacts from varying time periods all mixed up with each other.
Many people ask how archaeologists know when to stop digging? Good question! There are two common ways to determine when to stop digging. First, we stop digging a unit if we find two sterile levels in a row. Sterile means we have found no artifacts or features in a level so we can be pretty sure we’ve reached a point in the strata before any people occupied the site.
The second way is that we reach soil layers that are known to have been deposited before humans were ever in the area. Since we are studying humans, there is no need to dig before the point of human occupation.
Once we determine that a unit is finished, we often put in a water-permeable liner so archaeologists in the future will know the spot has already been excavated. At Fort Vancouver we put a penny minted in the current year in the bottom to show when it was excavated. Then we shovel all the dirt and rocks back in!
In the next post find out what we do with all those artifacts we find in the field! Plus, what other things do archaeologists do?
