LEARN: What is the process of Archaeology?


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.

Excavation Methods - 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.

Placing Excavation Units

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, 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 2x2 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 1x4 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 1x2 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.

We used the Total Station again to precisely align the excavation unit corners with the site grid, just like we did when we created the 20x20 meter squares for geophysical survey. The excavation units are created with nice straight sides and neat corners. Each excavation trench is outlined in bright string wrapped around metal stakes in each corner. The precise shape helps align the trench with the grid, and enables us to control where and how we recover cultural materials.

Geophysical Survey - Seeing underground

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 20x20 meter squares laid out precisely according to the Aztalan Grid.

There are many different geophysical instruments that can be useful in archaeology. We utilized magnetometry and ground penetrating radar. 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 excavation blocks.

Our results showed several "anomalies" that might be features like 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.

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.

What are we looking for? 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.

Laboratory Work