I recently completed a project near Moorhead, MN that involved pulling deep soil cores to check for palaeosols. Moorhead is situated on the ever-exciting Red River of the North. As the Red River floods and changes course, sometimes she will drop several feet of mud and sediments, burying whatever was on the ground when the flood moved through. When situations are just right, the mud can seal in an ancient ground-surface. When a surface soil is buried underneath such sediments, it is called a palaeosol.
As you can imagine, palaeosols are a bit of a holy grail for archaeologists. If we are lucky, we might find (for example) a 5000 year old campsite, looking much like it did when the flood hit. In 2008 in Crookston, MN (on a tributary of the Red River) I found a palaeosol 2.8m below the current ground surface. Ceramics and artifacts in that palaeosol dated it to the 1920s. Interestingly, we found buried soils about the same age throughout Crookston, in some places as deep as 4m below the current ground surface.
Like all holy grails, palaeosols (especially ones thousands of years old) are rare and difficult to find. What the Red River gives, the Red River takes away. In other words, to deposit mud on one spot (perhaps creating a palaeosol), the Red has to scour sediment from another spot (perhaps erasing a palaeosol). To compound difficulties, palaeosols can be quite difficult to identify. I love looking for them on the Red River because they tend to be easy to see—a dark layer of soil bedded in pale grey and tan sediments.
The best way to find palaeosols (or document their absence) is without doubt a backhoe. But backhoe trenches are frequently not an option. As a reasonable substitute I often take deep soil cores evenly spaced on a transect. The spacing depends on the local geomorphology. On a uniform plain, 50m spacing is pretty normal. On winding river banks, 15m is better. If I suspect palaeosols, 5m is called for. The spacing of cores is always difficult; budgets and timelines rarely allow the close interval spacing I prefer. The harsh reality of cores is that we miss most things. The goal for a compliance project is, however, to find any palaeosols with a sufficient horizontal distribution to contain substantial archaeological materials. For most review agencies, that approach is considered a reasonable effort.
There are a variety of methods that can be used to pull cores. When searching for palaeosols it is essential, in my opinion, to use a method that retrieve continuous cores in a liner. The cores, after all, are only a tiny fragment of what lies below ground, and thus the best possible sample is needed. I have created my own set-up, which mostly uses parts from a JMC Environmentalist’s Sub-Soil Probe. We have pulled cores more than 20 feet deep using this rig, and I can attest that the JMC probe is far tougher than you might imagine. While my setup is not as substantial as a Geoprobe, it has its advantages, including lower purchase cost, lower maintenance costs, and super portability (and more exercise). With an experienced crew, we move about 2/3rds as fast as a Geoprobe. Driving the core is the easy part. Getting it out is harder. The method I am using relies on a Jin-pole, polypropylene rope, and a Prusik knot. I learned this technique from Dr. Eduard Reinhardt, McMaster University. [Before you rush out and build your own, be forewarned. We are applying 200-500lbs of force (sometimes more) to extract the tube. Figure in the angle on the Jin-pole, the pulley, and the winch, and you have more than enough power to make things go horribly, horribly wrong.]
Here is some video of the coring in Moorhead:
Oh, and what did we find in Moorhead? Nothing but Glacial Lake Agassiz. Good for news our client, a bit boring for the field crew.