In order to properly conduct archaeological research, certain practices are necessary in order to regulate excavation methods, research goals, record-keeping, and information processing throughout the duration the project.
Our study site involving field excavation, Aztalan, is a perfect example for learning about what an archaeological excavation is and what archaeological research entails.
- Research Question
- What are we looking for?
- The Aztalan Grid
- Geophysical Survey
- Placing Excavation Units
- Excavation Methods
- Laboratory Work
Aztalan – A Diverse Community
Aztalan was a diverse community. At least two different cultural groups lived at the site between about 1050 and 1200 AD, from at least two different regions of the Midwest. Archaeologists call those groups Late Woodland and Mississippian. The Woodland people of Aztalan were local to the area, or at least had lived there for several generations, but the Mississippians were newcomers from further to the south – likely from an ancient city called Cahokia near modern-day St. Louis.
Mississippian and Woodland people lived differently in many ways – for example, they each had their own styles of pottery and their houses looked different and were built using different methods. These and other differences are visible archaeologically by the materials the people have left behind. By studying material remains from Aztalan – not just pretty artifacts, but also what’s left of their houses, garbage pits, and food – we can learn a great deal about who the people of Aztalan were, how they lived together, and maybe what happened to them.
Ancient Structures and Evidence of Daily Life.
The primary targets for excavation are three structures that were originally discovered and partially excavated by Joan Freeman and the State Historical Society of Wisconsin (SHSW) in 1964. As you can see in this image, the three structures have different shapes and construction methods – two are rectangular, one is likely circular, two were built using wall trenches, and one was built using single-set posts (posts erected into individually-dug holes). The combination of these different structures shapes and construction methods might be evidence of the daily interactions between Mississippian and Late Woodland peoples. We will relocate the 1964 excavation trench using a combination of archival maps and geophysical sensing, and will place our excavation units to uncover the intact, eastern halves of the structures.
We expect to find evidence of daily life inside the structures. SHSW found pieces of pottery and broken stone tools, so we also expect to find those types of artifacts. We also expect to find many things that SHSW didn’t keep during their excavations, like charred seeds and nutshell, animal bones, small flakes of stone from the manufacturing of tools, and charred wood. These kinds of remains are useful for understanding the activities that took place inside and around the household.
Maintaining Order Across the Site
One of the first steps of conducting an archaeological excavation is to establish a system of coordinates across the site. Creating an imaginary grid over the site allows archaeologists to describe the locations of their excavation units, artifacts, and features very precisely. This then allows other archaeologists in the future to locate exactly where those excavations took place. Grid systems need to be tied in to known points that can be easily found in the future. Three permanent benchmarks were placed in cement across Aztalan around 1950, which established an “Aztalan Grid” across the site. The grid is invisible, but can be located and recreated using the permanent benchmarks. The Aztalan Grid has been used by archaeologists working at Aztalan since they were created, which has enabled us to compile the precise locations of over 60 years of excavations.
The first thing we did this week was use an instrument called a “total station” to mark precise points along the Aztalan Grid in our area of focus. A total station is a computerized device that fires a laser toward a hand-held reflector. When the laser bounces back, the total station can calculate distance, angles, and elevation with great accuracy. By moving the reflector around the site, we can locate the precise locations of points on the grid, or other coordinates like latitude and longitude. We are using wooden stakes in the ground to mark the grid points we’re most interested in. When we use those stakes to place our excavation units next week, we know they’ll be aligned with the grid and that future archaeologists will be able to find them.
Before starting excavation, we worked with Dr. David Anderson of UW-La Crosse to do geophysical survey over two areas inside the residential area. These two survey areas have seen very limited excavation, so we know almost nothing about them. Geophysical survey can give us information about broad areas without needing to rely on very large horizontal excavation blocks, saving time and preserving as much as possible. Our goal for these surveys was to learn more about the spatial layout of the residential area, and to identify possible structures for targeted excavation. Our geophysical surveys were organized into ten 20×20 meter squares laid out precisely according to the Aztalan Grid.
As of 2017, we have utilized both magnetometry (in partnership with Dr. John D. Richards and Dr. Jarrod Burks) and ground penetrating radar (in partnership with Dr. Lynne Goldstein?) geophysical survey methods at Aztalan. Magnetometry works by detecting small-scale fluctuations in the earth’s magnetic field that are caused by things underground. Ground penetrating radar sends radar waves into the ground, which then bounce off boundaries between different soil types that might be the remnants of ancient activities, or off large features like burials, brick walls, or house foundations. After completing a survey, readings are stitched together to create two-dimensional maps of underground features over large areas. Similar investigation through excavation would require huge, invasive excavation blocks.
Our results showed several “anomalies” that might be features, such as house structures, storage or garbage pits, and stockade walls. As we move into the excavation phase of the project, we will place test pits over some of the anomalies to “ground-truth” them – that is, excavate to see if they match our geophysical interpretations.
Deciding Where to Excavate
Excavation isn’t done just anywhere in a site – locations are carefully selected to meet specific goals. It’s also unethical in most cases to excavate an entire site. This is because methods of excavation and analysis will surely continue to improve in the future, and new research questions will be asked that we haven’t even thought of today. It’s wise to only excavate places that will be useful for addressing the project’s research goals, leaving the rest for future investigations. Finally, excavation is slow, so archaeologists need to have reasonable expectations about how much they can accomplish within the time frame of the project.
Given these considerations and constraints, in 2015 we selected three initial locations to place four excavation units with the possibility of later expansion. The first location is the spot of a geophysical anomaly that matches the size and shape expectations for a house. Given the size of the anomaly, we placed two 2×2 meter square units to cover as much of the potential house as possible with the goal of determining the structure’s size, shape, and type of construction. The second area of excavation is also a geophysical anomaly, but this time appears to be a north-south running wall that is too long to be part of a structure – more like one of the palisade walls. A single 1×4 meter trench was placed perpendicular to the orientation of the anomaly so that if it is a wall, the excavation trench will cross it and we will be able to confirm its existence. The third location is the spot of the three houses originally discovered in 1964 by the State Historical Society of Wisconsin. A small 1×2 meter trench was placed so that it would pick up the 1964 excavation block in its west half and an intact structure in the east half. We chose a small excavation trench here because the state of erosion was worse than expected, and we are unsure if any portions of the three structures remain intact.
Precision, Paperwork, and Care
Professional excavation is usually done very slowly, for a few reasons: To maintain precision, to take extensive notes, and to be very careful.
Precision: A common technique is to excavate in 10 cm levels. Stopping every 10 cm gives the opportunity to record information and understand how the soil, artifacts, or other characteristics of the excavation unit might change as it is excavated deeper. Like carpentry, it’s good advice in archaeology to “measure twice, cut once.” Soil can’t be put back in the ground after its been dug, so an archaeologist will shovel off a thin slice of soil at a time and measure very often to make sure the target depth is reached without going too deep. The walls and floors of excavation units are kept very straight and neat. One reason for the precision is to keep good horizontal and vertical control. For example, if a level is supposed to be 10 cm deep, it should be 10 cm deep across the entire excavation unit. It also makes laboratory analyses and interpretations much easier because precise measurements are easier to describe, map, and photograph than irregular dimensions. Finally, keeping neat, straight walls with nice corners makes it much easier to view and interpret the stratigraphy (the layers of soil and cultural material) visible in the walls of an excavation unit.
Paperwork: Each level gets its own paperwork and notes that record the starting and ending elevations, how the level was excavated, what was found, what the soil was like, and how it might be interpreted. After a level is completed it is usually photographed and a precise map is drawn. The paperwork, maps, and photographs are critically important for making sense of things months later during laboratory analysis – perhaps even more important than the artifacts themselves. The reason paperwork and notes are so important is that an artifact loses a great deal of its importance if the contextual information about where it was found and what it was found with are lost. Extensive paperwork, notes, maps, and photographs ensure that as much information as possible is recorded, even if it seems irrelevant at the time.
Being careful: Archaeology is unfortunately a destructive process – to investigate a site, we must destroy it. As mentioned above, once soil is dug it can’t really be put back in the ground. Multiple complementary techniques are used in excavation to ensure that as much information can be gained as possible, but each technique takes time to complete. For example, soil is usually sifted through mesh screens to recover artifacts, but very small artifacts will pass right through the screen and be lost. This is a reality of archaeology – it is simply impossible to recover everything all of the time.
An excellent way to recover very small artifacts is a technique called flotation in which a sample of soil is bagged without screening. Later on in the lab, the soil is dissolved in water and some things like very small plant remains float to the top where they can be recovered through a very fine sieve, while other small artifacts separate out at the bottom. Flotation is impractical for very large quantities of soil, so the technique is usually done in conjunction with standard screening. Other techniques include taking charcoal samples for radiocarbon dating, soil samples for chemical, pollen, or microscopic analyses, using soil corers, etc. Every additional technique takes time to complete and most require additional paperwork. It would be fastest to only screen soil without using any additional techniques, but all the information gained through those other techniques would be lost forever. We can never know what kinds of information will be important in the future, so it is ethical to obtain as much information as is practical.
What Happens When Fieldwork Ends?
Every step of archaeological research must be handled with diligent attention to detail. This holds true out in the field and in the laboratory as well. However, in the past, archaeologists did not always keep to universal standards of excavation and artifact collection for the simple reason that up until the middle of the 20th century, no such regulations existed to help create uniformity within the field of archaeology.
One of the research projects our lab works on involves the Jonathan Creek collection from Kentucky, a large assemblage of artifacts which was uncovered in the early 1940s. The project was abruptly cut short at the start of WWII, leaving most of the artifacts uncleaned, uncatalogued, and often stored in questionable conditions. Here at the Wisconsin Archaeology Lab, we work to properly curate these artifacts in an effort to standardize the quality of artifact storage and cataloguing, as well as make the collection suitable for organized research.
The artifacts we excavate from Aztalan receive much the same treatment, though with a different cataloguing system. While excavating, each feature, such as a post mold or level, is given its own feature number, or FN. Paper artifact bags are assigned to each FN, and only artifacts from that feature go in its bag(s). Back at the lab, this sorting system comes in handy. We clean the artifacts in a single feature on one portion of a drying rack. Once they are dry, we sort the items by type (ex: shell-tempered body sherd, unburned animal bone, charcoal, etc.), and then make separate bags for each of these type groups within a feature. Once each group of artifact type is weighed and counted, they are placed within their labeled bags, which are then put into a larger bag which holds all artifact bags from a single feature. This way, we know how many and what type of artifact came from each part of an excavation unit.
Another important element of post-excavation work includes synthesizing the information across all field notes and creating graphs, maps, and visual aids to understand the spatial layout of the features uncovered. For our research at Aztalan with its focus on residential life, this step is crucial for understanding life patterns through construction history. We closely record soil stains representing structural foundations as the excavation progresses, continually making profile and plan view maps as the shapes change with depth. These maps showing layers of features can tell us when structures were built and when they were taken down, what types of structures were contemporaneous with each other, who built the them, and even how the structures were built.