By Justin Ayers
Howdy! Justin Ayers here, excited to explain the ongoing bucket auger testing at Sayles Adobe. You may ask what exactly is an auger? The auger is a simple but effective tool for collecting/sampling sediment below the surface without opening excavation units. It is comprised of a lengthy pole with a helical bit (and cylindrical bucket) at the end, which is designed to twist through the earth easily, while the bucket simultaneously collects the sediment displaced by the twisting bit. After one and a half turns of the auger handle, the auger bucket is filled with compressed dirt, and the device is gently pulled up out of the hole and the bucket load is dumped into a screen, examined, and recorded. The process is repeated over and over up to a depth of 3 meters (about 10 feet!).
Conducting a bucket auger survey of Sayles is important because it is allowing us to relatively quickly sample the stratigraphy across the terrace, a task that would take months to accomplish using hand excavation units. Further, by combining the different sets of auger data we are able to map out the subsurface deposits (both natural and cultural) of Sayles. These data will contribute to Tori Pagano’s ongoing thesis research (see Tales of Sayles Adobe), as she aims to define and sample the natural and cultural deposits of the terrace.
Bucket Auger Process
Tori’s plan is to build on the ground-penetrating radar (GPR) survey conducted at the site in January by Tiffany Osburn. We wanted to follow the GPR grid with our bucket augering as closely as possible so we could investigate several interesting radar anomalies seen at certain locations and depths within the terrace. Happily, our collaborator geoarchaeologist Ken Lawrence kindly allowed us to borrow his bucket auger!
We wanted to use the bucket auger tests to begin to “ground-truth” the GPR anomalies (i.e., determine what the anomalies are caused by). We also wanted to compare the sediment sampled by the auger to the exposed stratigraphy in the “Borrow Pit” excavation area. The auger testing is showing us that the stratigraphy of Sayles Adobe is more complex (and interesting) than that seen so far in our initial excavation exposure in the Borrow Pit.
You can see the bucket auger on the left, leaning against a tree. To the right you can see the rest of the equipment we used during auger testing: Munsell color chart, 2mm geologic sieve, 5-gallon bucket, and lots of recording forms!
Step 1: Stake out targeted locations of auger columns.
- Transect sampling intervals – 4 meters apart
- Shoot in surface points with TDS (Total Data Station)
Step 2: Begin sampling
- The auger excavates a 10cm diameter hole, and each bucket full of dirt is ~8cm deep. Once a bucket of sediment is collected, the auger is pulled up to the surface.
Step 3: Sieve the sediment.
- Once the bucket is pulled out of the auger hole, it is dumped into a 2mm sieve. Only the fine sand/silt mixture will pass through the screen, leaving snail shells, roots, FCR (Fire-Cracked Rock), and debitage on the screen.
Step 4: Documentation
- A metric tape (or “pocket stadia”) is used to measure the depth of the hole after each bucket auger sample was removed. This allows us to monitor our progress and make the auger data comparable from one auger column to the next.
- The sediment texture varies from a fine sand to a compact clay-silt.
- Color is recorded using a Musell color chart. We found that the 10YR color sheet works best for us.
- Lastly, general notes are made of the sediment, e.g., organic materials if present, FCR, charcoal, & ped toughness (Wikipedia: ped = a unit of soil structure such as an aggregate, … block, or granule, formed by natural processes.)
Step 5: Homogenize and Sample
The sediment that falls through the screen is homogenized (stirred around) in the bucket before we collect a small representative sample. After the field season Tori, under the tutelage of geoarchaeologists Dr. Charles Frederick and Ken Lawrence, will process the samples to determine grain size and test for magnetic susceptibility (among other things). Tori plans to use the auger column data to create composite stratigraphic profiles (sections) across the terrace that will be integrated with GPR and other lines of evidence.
I should note that while the bucket auger does its job very well, it mixes the sediment in each bucket load together such that thin layers only a few cm thick are hard to detect. And even though we are careful to insert and remove the auger gently, inevitably there is some admixture of sediment from the walls of the hole. In other words, we end up with somewhat averaged samples in 8cm increments. We homogenize each sample to make sure it truly represents an average of each increment. By carrying out each auger test in exactly the same systematic way, we can compare apples to apples with our auger samples and differentiate significant stratigraphic changes.
Luckily Sayles Adobe has very nice silty and sandy deposits (as opposed to compact clay or gravel) and testing has gone without a hitch … for the most part. Sometimes we are impeded by rocks and roots, forcing us to move our auger test column to a new location. If we encounter something hard that we can’t get through, we simply move the auger hole 30-40cm over from the original location and try again. Overall, we able to get down to the full 3-meter depth in only about 50% of our tests. In the other half we are stopped by burned rocks or other obstructions (like large mesquite roots) before reaching maximum depth.
What Are We Finding?
Based on our initial excavation in the Borrow Pit area along with the GPR survey findings, we suspected that we would hit the upper mud drape (I will explain why I say “upper” in a bit) around the same depth at multiple points on the terrace (see below profile of the Borrow Pit in Sayles).
So far we have found that the mud drape is not a perfectly flat horizontal layer, it tends to follow the topography of the deposits it covers. What I mean is that in places there may have been exposed rocks or humps in the ground that the mud drape settled over during the flood event, which may be the AD 1340 flood that we documented at nearby Skiles Shelter and Kelley Cave. Many (all?) of these rocks are FCR that were capped by the mud drape. The upper mud drape is thin and hard (clay-silt), compared to the fine sand and sandy silt that makes up the majority of the site’s deposits. In essence, the mud drape seals the cultural material that lies beneath it.
Since the mud drape covers the top of the uppermost cultural layer at Sayles Adobe, when we reach the upper mud drape the cultural layer should be directly beneath it. However, this cultural layer contains many FCR and often the bucket auger is stopped by the rocks. The auger will dig through most Sayles Adobe sediment quickly, but when you hit a sizable rock … everything stops. This is not considered a bad thing though. Usually when we hit a rock, we bring up chips of said rock that reveal if it is FCR. The FCR fragments tended to smell of sulfur when broken by the auger bit. The FCR encountered about one meter below the surface tells us that we are hitting the upper cultural layer … jackpot!
So far we have not created schematic stratigraphic profiles for all the auger tests, but we created a preliminary illustration based on a test from the east side of the site. The stratigraphic patterns in this auger test compare well to the stratigraphy observed in the Borrow Pit and Sandbox excavation, so we are anxious to complete augering of the entire site so we can get a better map of the stratigraphy!
Looking back at the GPR research done in January, the upper anomalies seen appear to have been the upper cultural layer. The FCR are in a large enough concentration to show significant feedback from the GPR. Three of our west terrace auger tests were stopped at a depth between 80cm-1m when we hit rock—usually FCR, which the auger could just not dig through. In fact, the Sandbox excavation area was laid out as the result of our first east-west bucket auger transect. Next week we should expose the concentrated FCR and see why so many burned rocks are piled up in one area.
Plans for the Future
Auger testing at Sayles is an ongoing process, with more sampling columns on the way. So far, only an east to west transect has been completed. We just started on our north to south transect, targeting more anomalies from the GPR survey. If additional FCR/cultural layers are encountered, it is likely that more units will be opened up for further research. The emerging picture from the bucket auger data is proving to be quite informative and tantalizing. Hint, hint: we have encountered deeper layers of mud drape silt and of cultural material yet to be exposed!