Finally, it’s time to get your hands dirty (after some planning)! In this chapter, we explore excavations—the “digs” alluded to but rarely shown in movie and television depictions of archaeologists (who always show up just in time to discover the treasure). This chapter explores the process of excavation and how it relates directly to implementation of archaeologists’ research designs.
Perhaps the first truly scientific excavations, directed by specific questions about the past, in the United States were conducted by Thomas Jefferson in 1784 in Virginia when he dug a trench in a burial mound to discover who had made it and why. This excavation allowed Jefferson to collect data that pointed to Native Americans as the mound builders and indicated that they had used the mound on multiple occasions.
Excavation is not an easy task. First, it is an expensive proposition in terms of time and financial resources. More importantly, however, it is a destructive technique since the archaeological record is not renewable. If an error is made during the excavation process, the archaeologist cannot undo that work or even redo it—what’s been dug up stays dug up. It is critical that nondestructive methods are used whenever possible and that excavation is used only when there is no other way to gather the data needed to accomplish the research goals.
Excavations can be small in scale, such as the 1-meter by 1-meter test pits discussed in earlier chapters, or large in scale, such as entire villages. The scale of excavation units for a study is selected based on the types of questions the researcher hopes to address. And initial excavations can answer some research questions, potentially requiring researchers to change the scale or even nature of their future excavations at a site. One approach archaeologists can use in such a case is vertical excavation, in which trenches or test pits are used to determine the depth of the time scale in the archaeological record. They examine the stratigraphic profiles and the artifacts deposited in the layers to see whether the culture changed over the course of time during which the site was occupied. In contrast, a horizontal excavation exposes a large, relatively shallow area to understand larger site-configuration and function questions. Typically, horizontal excavations are used to study large-scale regional areas to understand how use of the environment differed across space. Horizontal excavations usually are not as deep as vertical excavations because time depth is not a critical component in such studies.
Stratigraphy, the study of layers of soil, is an important component of all excavations but particularly critical for vertical excavations. Stratigraphic data assist archaeologists in putting the archaeological record into context; the data provide a relative way to date the site and its contents and can provide some contextual clues about natural formation processes that occurred after the site was abandoned. For stratigraphy to be used scientifically, the researcher must make two assumptions, both based on the work of Nicolaus Steno, a geologist from the seventeenth century. The first assumption is that soils accumulate in layers that are laid down parallel to the Earth’s surface. This is known as the Law of Horizontality. The second assumption, the Law of Superposition, assumes that older soils will typically (but not always) be found below younger soils (that the old stuff will be on the bottom). These two assumptions allow archaeologists and others using stratigraphy in their work to understand how the soils accumulated and to use the layers to “tell time.” Archaeologists often are on the lookout for a marker horizon when studying stratigraphy. Marker horizons are distinct layers, such as a layer of ash between layers of clay, that provide additional context to the stratigraphic profile or story.
Once archaeologists determine that an excavation is needed to answer their research questions, they must tackle several steps. The first step is to map the site and create a grid system that they base on the coordinates of a fixed point, the datum, which is used for all of their future measurements. The datum typically is a prominent geographic feature of the site such as a large boulder, building, or fence post to which a GPS point can be affixed. Using an immovable object as the datum point allows future researchers who excavate in the same area to make reference to earlier work.
After the site has been mapped, an excavation method is selected (if not already determined in the initial planning phases). The archaeologists consider whether to dig trenches or deep pits in a vertical excavation or a relatively shallow horizontal excavation over a large area of the site. Other excavation choices can be dictated by the archaeologists’ training. For example, most archaeologists who excavate in the United States do not use the Wheeler box grid method of excavation commonly used in other countries. In that method, intact baulks (walls) are left between each grid-square unit so the stratigraphy of the site can be more easily “read.” The baulks are sometimes removed at the end of excavation. The most common method used in the United States is open area excavation with no baulks left between units; the grid squares adjoin each other and are entirely cleared. Often, the natural geography, strata, and/or cultural layers dictate which excavation method is used.
When the excavation method has been selected, actual digging can begin. Archaeologists are systematic when they excavate since, as mentioned earlier, archaeological data cannot be renewed.
You have probably seen photographs of excavations, in this text and in other texts and publications, in which the “holes” are square rather than round. Why does the shape of the hole matter? By digging a square hole, archaeologists can easily calculate how many artifacts and other items are present per unit—in this case, a measure of volume. Since a square is made of up two equally sized right triangles, archaeologists ensure that the holes they dig are perfectly square using the Pythagorean Theorem: a2 + b2 =c2. Using this calculation when the initial grid lines of the map are drawn and when individual units are established will ensure that each archaeological unit is a perfect square.
The next decision, likely made before excavation begins, is how deep each level will be since the excavation is in three dimensions—length, width, and depth. Some archaeologists elect to tie the depth of each level to the natural strata of the site with each layer representing a level. More often, though, archaeologists select an arbitrary strata depth such as 10 or 20 centimeters regardless of the stratigraphic layers.
When excavators reach the bottom of a natural or arbitrary strata level, several things occur. First, the archaeologist typically takes measurements of the depth of excavation across the entire square to ensure it was excavated to precisely the same depth throughout. This is important as the surface, where excavation begins, is typically naturally uneven, but the bottom of the strata level should be flat. And because the ground is rarely level, a plumb bob or line level is often used when taking the measurements. The archaeologist and workers then draw sketches of the excavated layer and its stratigraphic profile and photograph the entire unit, the stratigraphic profile, and important characteristics of the soil to document the stratigraphy. One photograph documents the unit’s location on the grid system and the depth of the layer excavated using a sign and a tool such as a trowel pointing north to make it easy to orient the unit and identify its location later.
This process continues until the archaeologists have gathered all the information they can from the unit, encounter something unexpected (such as the water table), or come to the end of the project. Before the unit is backfilled, they take high-quality photographs and draw sketches of it. Sometimes, before backfilling a site, the archaeologists put something modern, such as a contemporary soda can, at the final depth excavated. This indicates their stopping point to archaeologists who resume excavation there in the future. The depth and, of course, the marker are indicated in the archaeological report.
As excavating proceeds, the material removed is sorted using a screen. The screening method used varies with the circumstances, including the type of matrix or soil and the artifacts or other archaeological remains they expect to uncover. Typically, screens consist of a wooden frame with window screening material affixed to the bottom. The soil is put into the screening box, and workers sift the soil through the screen, which leaves larger chunks and objects behind. The screens’ dimensions vary based on the artifacts of interest to the archaeologist. The mesh’s hole size can be anywhere from one-half to one-sixteenth inch.
When archaeologists are especially interested in locating pollen or other small plant remains, they can use a water screening process called flotation, in which the excavated material is flushed through a water sieve that allows the lighter materials to float to the surface, making them easy to recover. Water screening is also sometimes used for larger objects encased in a matrix that is primarily clay or some other dense or wet soil. In that case, hoses or buckets of water are used to wash the dirt off the objects.
Decisions about whether to use wet or dry screening methods and the size of the screen to use have a dramatic impact on the kinds of artifacts that can be recovered and the condition in which they are retrieved. Small artifacts will be lost to time if the screen is too large, and pressure from a water hose can damage or even destroy fragile artifacts. So these seemingly small decisions are a critical part of planning an excavation.
During the screening process, workers sift the materials and pull out artifacts and ecofacts. Each item is preliminarily put in a bag clearly marked with its provenience (three-dimensional coordinate information, including its layer and its specific position relative to the surface—the depth at which it was found). Eventually, a field catalog will be created that records everything that was uncovered in the field. Both identification of the finds and creation of the catalog will be refined once the team returns to the lab.
It is important to realize that there is a lot of work to complete after excavating a site—many archaeologists would argue that most of the work is done after they return from the field! A common estimate of the breakdown in time spent in the field versus the lab is 1 to 5. For every 1 week spent in the field excavating, archaeologists expect to spend at least 5 weeks in the lab processing what they found. Some archaeologists wash the artifacts when they return to the lab; others prefer not to wash them because they are interested in obtaining DNA or other types of trace materials for analysis. Each artifact and piece of archaeological material is given a catalog number that corresponds to a listing of it in the permanent catalog, which is built from the field catalog. The permanent catalog, which commonly is a computer database, records the provenience data for the item and a brief description of it and often includes a photo. The catalog number is written on the artifact in permanent, archival ink. These numbers need to be written legibly in an inconspicuous but noticeable place. So, obviously, not across the face of a Pakal’s mask! With such a mask, the catalog number would likely be written on the inside surface of the mask. The items are then placed in bags with the provenience information written on the outside of the bag and, usually, on a small tag placed inside the bag. This seeming duplication of information is critical in case the information on the bag rubs off or something happens to the number on the artifact.
The process used for underwater excavations is somewhat different for obvious reasons. Underwater excavations usually are not designed to recover the artifacts; rather, the task is to record all of the artifacts visible on the lake or sea floor. It is difficult and costly to bring artifacts to the surface, and, once exposed to air, many artifacts, such as metal encrusted with metallic salts, must be stabilized in a laboratory to prevent them from deteriorating. Miniature submarines, submersible watercraft, scuba gear, and other technologies allow archaeologists to see below the surface.
Side-scan sonar can also be used to locate shipwrecks and other archaeological sites beneath the surface. Side-scan sonar is similar to LiDAR; sound waves are sent to the ocean floor, and the scanner measures how long it takes for the sound waves to return, creating a topographical map of the floor’s surface. Side-scan sonar mapping is expensive, however, and a somewhat less sophisticated sonar technology used on fishing boats to locate fish has been used successfully in archaeological expeditions.
When underwater archaeologists do collect artifacts and bring them to the surface, additional technologies such as suction hoses and baskets with balloons attached are needed. The tools used depend on the depth of the water and the types of artifacts being removed.
Terms You Should Know
- arbitrary strata
- field catalog
- horizontal excavation
- Law of Horizontality
- Law of Superposition
- marker horizon
- natural strata
- open area excavation
- permanent catalog
- Pythagorean Theorem
- side-scan sonar
- vertical excavation
- Wheeler box grid
- Describe some of the pitfalls an archaeologist could encounter during an excavation. What can archaeologists do to avoid those pitfalls?
- How are horizontal and vertical excavations different?
- When and why would flotation be used as part of the screening process?
- Why are assumptions made about the formation of stratigraphy important for archaeologists?
- What are the roles of the field and permanent catalogs? How are they created, what kind of data do they store, and how is the data used later in the lab?