Soiled! Part 3

Another other useful “behaviour” of soil is how it compacts, which can be recorded to differentiate contexts and predict how drainage in the area works! Compaction is incredibly important in modern agricultural practice as it can affect the structure of the soil, which in turn determines how long water is retained. Compact soils also make plant growth more difficult because roots are unable to absorb the required nutrients.

The compaction of soil is how much space is between the particles, and therefore it can be measured as density (how much stuff is packed into a known space). The spaces between the particles are called pores, which can hold air and water. Causes of compaction range from heavy farm equipment (not really a problem in our trenches as no one was farming in them and tractors were not A Thing) and regular treading by foot, to something as small as a raindrop.

Martin Watts (Mill Archives, 2016) after the Luttrell Psalter, Add MS 42130 at the British Library (circa 1340)

Processes to break compacted soil can be artificial or natural. The earliest means of turning soil involved wooden and bone or antler hoes and stone-blade hand-implements. With the advent of farming and associated animal domestication in the Neolithic period, animal-drawn ploughs first appear in Mesopotamia and the Indus River Valley. In Uruk (Mesopotamia, now Iraq), the earliest visual representation of ploughs appears around 3000BCE! Natural means of loosening compact soil include things like plant root growth, animal burrowing (“bioturbation”), and freeze-thaw cycles.

Archaeological records usually employ some standard terminology described by the Museum of London Archaeology Service when it comes to compaction. Their system differentiates between coarse-grained sediments, finer soils, and peat.

Coarse-grained sediment are deposits like gravels and sands, which in general have angular edges to their particles. They can be varying degrees of cemented, requiring larger tools like mattocks and picks. Compact sediment can be excavated with mattocks and heavy troweling. Loose soil is easily moved simply with basic troweling.

Finer sediments include silts and clays which take a long time to settle out of moving water. For these sediments, they use a hardness to softness spectrum, and another bit of vocab: friable. The spectrum includes hard (unable to mould), firm (difficult to mould), and soft (easily moulded). “Friable” means it’s not mouldable because it crumbles immediately.

Is this silty clay or clayey silt? (USDA)

Peat, which we unfortunately don’t get here, has three categories: firm, spongy, and plastic. The first two are pretty self-explanatory, but “plastic” in this sense means extremely mouldable, squishable, and smeary.

Peat. (Peatlands.org)

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