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**OBION** · 05-03-2016, 12:18 PM

Wow, that is some patina!!

**painshill** · 05-03-2016, 03:02 PM

Soil Science was one of my modules at University. Here’s a few things for you to think about (some of which I believe I have posted previously in response to someone else’s questions about stratigraphy).

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.

In non-volcanic areas such as Britain, the uppermost ‘A’ horizon is derived mostly from the weathering and frost-shattering of rocks (sands, silts and clays), supplemented by decaying organic material. It’s deposited by wind or water (and in other areas by glaciation and/or volcanic ashfall), supplemented from biological activity of both plant and animal organisms. In 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 actual observed increase is smaller.

The variation around these numbers can be huge, depending on the nature of the geology, water-courses, climate, altitude, vegetation and occupancy (both today and in the past). However, there is a tendency to think of soils as “static”… or only dynamic in the sense that they simply carry on accumulating in depth by largely natural geologic processes unless there has been human interference (river management, ploughing, construction or whatever).

Nothing could be further from the truth! Soils are highly “dynamic”, and artefacts within the strata are subject to both lateral and vertical displacement from non-geologic activity. It’s called “bioturbation”. In Britain, earthworms and ants have a lot to answer for, as well as borrowing vertebrates such as rabbits, foxes and badgers. In other countries, you can include termites, gophers and (interestingly) crayfish. Burrowing and mounding by these animals tends to displace larger objects downwards and smaller objects upwards. Tree-uprooting, (whether natural from weather effects or human from land clearance) tends to displace everything upwards. Then you have non-biological processes such as freeze-thaw and shrink-swell. As long ago as 1896, Charles Darwin’s experimentation recorded a one-fifth of an inch movement of a stone during a single wet-dry cycle of a typical soil.

Here’s the (heavily edited) discussion and conclusion sections from Cynthia Balek’s interesting paper published in “Geoarchaeology” in 2001, entitled “Buried Artifacts in Stable Upland Sites and the Role of Bioturbation: A Review.”

Discussion
The crux of this paper is to impress upon those who participate in archaeological […and other] work that biomechanical processes impact “stable” upland, nonaccreting archeological sites disrupting the original archaeological patterns (Schiffer, 1987). Artifacts can be sorted by size, become temporally mixed, and can be displaced laterally and vertically… The external processes are succinctly summarized by Michie (1990)…
Whenever artifacts are buried, we have been trained to think in contemporary geologic thought and consider the processes responsible for burial. More often than not, we look for evidence of fluvial deposition, aeolian sands, colluvium, or any combination of these processes.
Furthermore, we’ve been trained to believe the age of the artifacts within the deposit is a reflection of depositional antiquity. For example, if we find Late Archaic cultural material within a deposit, we naturally assume that the associated soils were deposited sometime during or after the occupation. We are encouraged to think this way because many sites have been buried by a series of dynamic processes. We have only to look at deep site profiles along major streams and see the layers of silts, clays, sands, and other sediments to be convinced that rivers breach their banks and deposit sediments along levees. When sand is being blown across dunes we realize the dynamics of wind, and when hills erode we see the accumulation of colluvium. Such examples are all around us, and consequently we learn to recognize the very processes that have shaped our world. It all seems to be related to erosion and deposition.
However, this emphasis on geologic processes in artifact burial leads to the misconception that soils developed on stable surfaces are static entities (Atkinson, 1957). We implicitly, and sometimes explicitly, reason that if erosion or deposition is negligible, then the surface is stable, and so is the soil. And, if the soil is stable, then any artifact dropped onto the surface will remain at the surface so long as deposition of new sediment does not bury it (e.g., Waters, 1992; Waters and Kuehn, 1996).
However, a stable geomorphic surface does not mean that the soil is stable or static (Atkinson, 1957). Internal biomechanical processes continually create a dynamic medium through which artifacts discarded onto the surface are readily displaced vertically and laterally. As early as 1957, Atkinson warned archaeologists that earthworm activity could alter the stratigraphic relationships of artifacts and that their ability to displace artifacts downward could not be ignored. Earthworms, however, are only one among many motile faunal lifeforms that cause bioturbation.

Conclusion
Burial of most, if not all, artifacts in stable upland soils developed in pre-Holocene sediments in nonfeature contexts, is due to vertical movement of the artefact in response to normal biologic activity, namely, burrowing and mounding by earthworms, ants, and other fauna, and by tree-uprooting. As long as soil biota continue to exist and create voids through which artifacts can fall, and as long as lifeforms continue to mix the soil matrix through preferential ejection of materials from depth onto the surface in the form of burrow mounds, or through tree uprooting, artifacts will be displaced.

In short, there are all kinds of reasons why an artefact with considerable antiquity and patina might end up in the plough zone (or even on the surface) and iron ploughs were in common use in England from the mid-1700s, well before Victorian times. Iron hoes and mattocks have an even longer history. Trampling (usually by horses or cattle) also has an often-forgotten capability to break lithic artefacts at or near the surface.