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Understanding Stratigraphy
Artifact Burial & Displacement in Soil Strata
Soils are composed of layers known as “horizons” – of which there can be many, but most soils have an ‘A’ horizon (topsoil with sand, silt and organic material), a ‘B’ horizon (subsoil with high levels of clay and inorganic minerals forced downwards by leaching) and a ‘C’ horizon (fragmented rocks and small stones with very little organic material). Then bedrock.
The uppermost ‘A’ horizon accumulates mainly from the weathering and frost-shattering of rocks plus dust from volcanic eruptions in many areas. It’s deposited by wind, water and glaciers, supplemented by decaying organic material. In very broad terms, in a temperate climate, it takes about 800 to 1,000 years for a typical fertile soil to gain an inch in depth. It actually happens faster than that, but the accumulated soil itself is also subject to erosion and typically loses around an inch in depth every 100 – 250 years such that the observed increase is reduced relative to what actually accumulates.
That’s a very simplistic picture though since the variation around these numbers can be huge, depending on the nature of the geology, water-courses, climate, altitude and vegetation (both today and in the past), generally further influenced by human interference. Some areas west of Mount St Helens gained 2 inches of topsoil in a matter of days after the 1980 eruption. The Great Plains lost an average of 5 inches of topsoil during the dust-bowl era of the 1930’s.
It’s also simplistic because even when soils are apparently “static”, they are still actually “dynamic”, such that artefacts within the strata are subject to both lateral and vertical displacement. Earthworms, ants, termites, burrowing vertebrates (notably gophers) and (interestingly) crayfish, as well as other factors continuously mix and displace particles. Burrowing and mounding tends to displace larger objects downwards and smaller objects upwards. Tree-uprooting, tends to displace everything upwards. Also, non-biological processes such as freeze-thaw and shrink-swell further influence the stratigraphy. As long ago as 1896, Charles Darwin recorded a one-fifth of an inch movement of a stone during a single wet-dry cycle of a soil.
Here’s the abstract from “Buried Artifacts in Stable Upland Sites and the Role of Bioturbation: A Review” by Cynthia Balek, published in “Geoarchaeology” in 2001:
Burial of artifacts in Holocene soils developed in pre-Holocene sediments on stable uplands is commonly interpreted as resulting from post-depositional accretion of sediment. However, soils are by nature dynamic and burial or displacement of artifacts can and does occur due to “normal” syndepositional and post-depositional biomechanical pedogenic processes. This paper presents a review of the role of bioturbation in artifact burial. [copyright 2002 John Wiley -and- Sons, Inc.]
You can download the full paper as a pdf file here:
Soils are composed of layers known as “horizons” – of which there can be many, but most soils have an ‘A’ horizon (topsoil with sand, silt and organic material), a ‘B’ horizon (subsoil with high levels of clay and inorganic minerals forced downwards by leaching) and a ‘C’ horizon (fragmented rocks and small stones with very little organic material). Then bedrock.
The uppermost ‘A’ horizon accumulates mainly from the weathering and frost-shattering of rocks plus dust from volcanic eruptions in many areas. It’s deposited by wind, water and glaciers, supplemented by decaying organic material. In very broad terms, in a temperate climate, it takes about 800 to 1,000 years for a typical fertile soil to gain an inch in depth. It actually happens faster than that, but the accumulated soil itself is also subject to erosion and typically loses around an inch in depth every 100 – 250 years such that the observed increase is reduced relative to what actually accumulates.
That’s a very simplistic picture though since the variation around these numbers can be huge, depending on the nature of the geology, water-courses, climate, altitude and vegetation (both today and in the past), generally further influenced by human interference. Some areas west of Mount St Helens gained 2 inches of topsoil in a matter of days after the 1980 eruption. The Great Plains lost an average of 5 inches of topsoil during the dust-bowl era of the 1930’s.
It’s also simplistic because even when soils are apparently “static”, they are still actually “dynamic”, such that artefacts within the strata are subject to both lateral and vertical displacement. Earthworms, ants, termites, burrowing vertebrates (notably gophers) and (interestingly) crayfish, as well as other factors continuously mix and displace particles. Burrowing and mounding tends to displace larger objects downwards and smaller objects upwards. Tree-uprooting, tends to displace everything upwards. Also, non-biological processes such as freeze-thaw and shrink-swell further influence the stratigraphy. As long ago as 1896, Charles Darwin recorded a one-fifth of an inch movement of a stone during a single wet-dry cycle of a soil.
Here’s the abstract from “Buried Artifacts in Stable Upland Sites and the Role of Bioturbation: A Review” by Cynthia Balek, published in “Geoarchaeology” in 2001:
Burial of artifacts in Holocene soils developed in pre-Holocene sediments on stable uplands is commonly interpreted as resulting from post-depositional accretion of sediment. However, soils are by nature dynamic and burial or displacement of artifacts can and does occur due to “normal” syndepositional and post-depositional biomechanical pedogenic processes. This paper presents a review of the role of bioturbation in artifact burial. [copyright 2002 John Wiley -and- Sons, Inc.]
You can download the full paper as a pdf file here:
