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Class Notes: Geoarchaeology

Friday: September 30, 2011

Week 4: Archaeological Soils & Correlations


We had a lab using Adobe Illustrator to take shovel test probes provided from real archaeological field notes and reconstruct the strata. As the name implies, holes are dug with a shovel to probe the landscape so as to test for the possibility of an informative feature. The width between the probes varies depending on the site – I have no idea in my representation below if there is a meter or tens of meters between them, but I am able to do a rough estimation of what lies below the ground with the data collected in the field notes.

Shovel Test Probe lab

Statigraphy correlated from shovel test probes done at an archaeological site.


Soil is different than sediments. Soil itself is weathered sediments that develop over a long period through a stable landscape. Sediments can accrue instantaneously, such as being deposited from a tsunami. Soils are only found on the surface, whereas sediments are found throughout the earth. If a soil surface becomes buried by sediments, it is no longer considered a soil, but instead a paleosol. There are five soil forming processes (also known as pedogenesis).

  • Climate
  • Organisms
  • Relief
  • Parent Material
  • Time
Soil also requires four components to be differentiated from sediments:
  • Inorganic mineral matter (derived from its parent material)
  • Organic matter (derived from the organisms)
  • Soil pore space (empty space between particles with allow water percolation and movement of organisms)
  • Soil water (some will fill the pore space, and others will be bonded to the aggregates themselves)
Soil characteristics for classification include:
  • Color (using the Munsell Soil Color Guide)
  • Texture (relative frequency of particle sizes)
  • Structure (shape of aggregates)
Soils can be described by their orders as well. We covered several:
  • Alfisol (found in upland environments, are older but with higher base and less weathering)
  • Mollisol (found in prairies and flood plains)
  • Entisol (thin layers, very young)
  • Inceptisol (slightly older than Entisol)
  • Histosol (found in marshes and swamps)
  • Oxisol (found in tropical rain forests)
  • Ultisol (like Alfisols, but older and the bases have been lost)
  • Aridosol (found in drylands and deserts)
  • Vertisol (found where clay shrinks and expands)
  • Spondosol (found in confiferous forests, highly acidic)
  • Gelisol (found in permafrost environments)
  • Andisol (volcanic soils)
Soil can be classified through taxonomy as well.  These are known as horizons. Thinking in terms or the surface on top, and going deeper, this is a typical Indiana soil broken into horizons:
  • O : If there is decay of organic material, there will be an O Horizon. Most soils lack this layer because bacteria acts quickly to dissolve it, but it remains where there is leaf litter, peat, or muck.
  • A : This is the most common top soil – it includes the highest concentration of organic matter (when O is not present) and therefore is darker and richer.
  • E : Sometimes there will be a small E Horizon. This occurs when water leaches the fines (clay sized particles), bases, and organic matter, leaving it lighter and sandy. It is most common in coniferous forests or in coastal environments.
  • B : Leaching has caused the loss of all bases, but the fines accumulate. This creates a reddish, more firm version of A due to a higher clay content.
  • C : This is the parent material layer  where the soil rests, and what is being weathered to become the soil, be it a deposit of clay or bedrock, though bedrock itself sometimes gets the designation of an R Horizon.
Tax dollars went into archiving US soils and providing a free website to learn about them (so get your money’s worth and check it out!). The Web Soil Survey allows you to type in an address, or simply zoom in over an area of choice. If you then click on the AOI button (the one with a red rectangle) at the top of the map, you can draw a box to create an area of interest. It will draw a box with diagonal lines. Then click on the Soil Map tab. It will give you a break down of the specific soils in that area. If you poke around some more, it provides all kinds of information. In addition to having fun on the internet, you can order any Soil Survey you create – for free. For instance, this is what campus looks like:
UIndy soil survey

Soil survey of UIndy. I spend most of my time in the basement of the building below the orange bullseye.

UIndy soil survey key

Soil composite of UIndy.

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