4.3: Worm Sinkage


Worm casts on grass (lawnhealth.co)
One other site formation process needs to be discussed: worm sinkage. This was first brought to public attention by the naturalist Charles Darwin, who wrote a book about it in 1881 that became very popular:  The Formation of Vegetable Mould through the Action of Worms, with Observations of their Habits. The book was the first scholarly treatment of soil-forming processes. In it Darwin points out the important roles worms play  including their role in sorting sediment, soil formation, decomposition of rocks, and improving soil conditions for plant growth by aerating the soil.

While it was long known that things left out in the open got gradually buried ("work themselves downards"), Darwin determined that this was due primarily to burrowing animals, in particular earthworms. Worms excavate burrows and these eventually collapse in on themselves, and over many years and centuries this reconfigures the ground surface above them. One of the mechanisms by which the worm not only burrows, but also gains nutrition is by consuming the soil and organic material that it contains. This is then excreted in the form of worm casts on the ground surface  and it is the slow accumulation of these casts on the surface that buries objects and structures.

In doing the research for his book, Darwin examined a number of archaeological sites. He describes an ancient Roman villa in Abinger, Surrey where worms had penetrated the concrete walls and even mortar. He also discusses Chedworth Roman Villa in Gloucestershire, Brading Roman Villa, Isle of Wight and also at some length the well preserved ruins of Silchester Roman Town, Hampshire and the Viroconium Roman town ruins at Wroxeter, Shropshire. He concluded that both worms and other causes, such as dust deposition and washing down of soil, have buried such ruins. The casts, being very fine soil, tend to erode downslope causing soil creep, the crowns and furrows of formerly ploughed lands slowly vanish when under pasture, due to worms.

Darwin determined that there were in some areas 50,000 worms per acre (124,000/ha)  and that the annual total weight of their casts was 18 tons. It is now believed Stewart 2005, 20-21) that this estimate was wrong and there could be as many as a million an acre (2.47m/ha). Worms can bring up 1–10 mm per year of soil and deposit it on to the surface.

The effects of earthworms on sites is described in an important (and in parts entertaining)  text by Canti 2002  who observes that many of the details of the archaeological effects of earthworms on archaeological stratigraphy "have not been fully grasped by stratigraphers or by the specialists who advise them" and have been little exploited from an interpretative point of view.

Worm-sorting of soil (Canti 2002, fig 7)
However lofty its ideals, archaeology is still a subject that is actually carried out in the realm of earthworms. Their burrows and galleries form and reform the matrix surrounding the harder materials from which we deduce whole cultures. Their activities push it, sort it, digest it and finally cast it, usually close to where it came from (hence we still have a subject to study), but sometimes further away.
The effects of worms sorting the soil can be seen in the layers of stones (including peagrit that is sometimes used to line the basal chamber of some worm burrows)  that can be seen in sections through soil that has been under grassland for some time. The constant burrowing under stones, artefacts and so on, and the formation of casts around and then above them as the burrows collapse and the undermined objects sink infinitesimally year by year mean that worm-sinkage is a major site formation process by which abandoned material moves vertically down 'through' the worm-sorted soil. The point is that most of the movement is indeed vertical, and horizontal relationships between the position of items originally lying scattered on the ground surface are preserved.

Similar effects are the result of the activities of other burrowing animals such as ants and moles, though these tend to affect the upper levels of soil, while earthworms can penetrate the subsoil by over a metre.

References:
Darwin, C. 1881, 'The Formation of Vegetable Mould through the Action of Worms, with Observations of their Habits' ,

Canti, M.G. (2003a). Earthworm activity and archaeological stratigraphy: A review of products and processes. Journal of Archaeological Science (2003) 30, 135–148

Stewart, A. 2005, 'The earth moved: On the remarkable achievements of earthworms', Chapel Hill, N.C: Algonquin Books

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