Issue 108

Sept / Oct 2009

Contents

news

Major slipware kiln site found near Leeds

Roman graves rescued: but cemetery doomed?

Isle of Man house is one of Britain's first

In the press

In Brief & Phase 2

features

THE BIG DIG: Fetternear
Penelope Dransart reports on the topical issue of MPs claims expenese, at Kettlethorpe Hall

London: the mud of ages
Lorna Richardson reports on the discoveries made by the Thames Discovery Programme community initiative and Nick Booth describes his Foreshore Group training

For the sake of the worms
As we celebrate Charles Darwin, Matthew Law considers one of his less well-known interests that led him to excavate at ancient sites

on the web

Recommended websites
Caroline Wickham-Jones reviews How to get active with archaeology, and John Schofield looks at Flash methods to view the evolution of graffiti

letters

your views and responses, with further Beneath the Sea coverage

book review

We review a new publication about the Vindolanda Roman Fort

CBA Correspondent

Mike Heyworth welcomes new HLF money for training, and highlights the CBA's role

ISSN 1357-4442

Editor Mike Pitts

features

For the sake of the worms

As we celebrate Charles Darwin, Matthew Law considers one of his less well-known interests that led him to excavate at ancient sites.

As is by now well-known, 2009 marks major anniversaries of Charles Darwin, including the bicentennial of his birth and 150 years of his most celebrated work, On the Origin of Species by Natural Selection. It is also 170 years since the publication of his Voyage of the Beagle, in which he described how, at the age of 26, he made observations as a naturalist on the survey ship that would later inspire his theories on evolution and natural selection, that still drive biological science.

Darwin never went on another expedition, as an unexplained illness plagued him after returning on the Beagle until his death in 1882. Rather, he spent much of the rest of his life experimenting at his home, Down House, at Downe, in Kent. He waited 20 years to publish his theories on natural selection, and only then because Alfred Russel Wallace had produced a manuscript with almost identical conclusions. Darwin was an innate naturalist, ever curious about biological processes, and took advantage of the sizeable grounds and greenhouses at Downe to conduct numerous experiments.

One thing that intrigued him was the effect that earthworms have on soil in altering the organic topsoil, or vegetable mould as it was known. His study of the action of earthworms was the subject of a paper presented to The Geological Society in 1837, published in Transactions of the Geological Society of London as a review the following year and in full in 1840; a note in Gardener's Chronicle & Agricultural Gazette in 1844; another in 1869; and finally a book entitled The Formation of Vegetable Mould, through the Action of Worms, with Observations on their Habits, published by John Murray in 1881. It was his fastest-selling book, and his last major work.

Darwin's uncle (and father-in-law) Josiah Wedgwood II (1769–1843) had shown him some fields at Maer Hall in Staffordshire that a few years before had all been covered either with lime or with cinders, but which, having been left undisturbed in the intervening time, had reverted to soil and turf. Darwin observed that on pasture land the top few inches of soil is of "uniform fineness", and explained that this can be seen quite clearly in gravelly areas where a recently ploughed field is contrasted with one that has been used for grazing. In the undisturbed field, the topsoil will be fine sediment with the gravel forming a line lower in the profile, whereas the ploughed field, as anybody who has been fieldwalking for artefacts after ploughing will recognise, is a jumble of gravel and finer sediment.

To investigate what he saw in Staffordshire, Darwin dug some small square trenches. He observed that the lime and cinder were all now buried at a depth of between one and three inches (3–8cm), forming a continuous line across the section. He realised that they could not have sunk there through their own agency, and thought that the change was too great to be the result of decay of the plants growing on the surface (which was the established belief at the time). Wedgwood thought that the casting activities of earthworms might be responsible. So Darwin set to work testing the theory, keeping worms in pots of soil to observe their behaviour, and, in 1842, spreading chalk over pasture land by his house in Kent – an early exercise in experimental archaeology.

In 1871, 29 years later, he dug a trench through the field, and found that the chalk nodules now sat in a line across his section at a depth of seven inches (18cm). A number of other trenches were excavated on land where material had been spread, and all showed that the material had been buried and formed a line. In one trench, two separate episodes of spreading had resulted in two parallel lines, one at a depth of seven inches, and another at a depth of five and a half inches (14cm).

Different species of earthworm eat organic matter in various states of decay, passing the waste as casts containing a mixture of organic and mineral components. Some species of worm travel to the surface to produce their casts, which can readily be seen as small mounds of earth on lawns. Most feeding by surfacecasting species occurs in the top few centimetres of soil where the organic content is higher. This is the reason for the lines of lime and cinder that Darwin observed in his sections in Staffordshire. Eventually, an object on the surface buried by worm casting will reach a level below which worms are not feeding, and so they will remain at a stable depth. Subsequent worm casts will be the result of reworking of the soil above that line.

Darwin also observed that worms bury larger objects by burrowing underneath them and casting alongside them. He reported the partial burial of stones from an old lime kiln at Leith Hill Place in Surrey as an example, and also the build-up of sloping borders of turf around the fallen stones at Stonehenge. At Stonehenge he dug a number of small holes to examine this build-up, and found lines of flint and, in one hole, a piece of broken glass at nine and a half inches (24cm) below ground level. He also dug a hole some distance away from the stones, and found there that the larger components were at a depth of five and a half inches (14cm), as well as seeing a fragment of clay tobacco pipe, evidently on its way down the profile, at a depth of four inches (10cm).

In 1877, a Roman villa was excavated at Abinger in Surrey, and Darwin was present to examine the soil. Initially, he attributed the burial of the remains of the villa to colluvial action, as the villa was at the bottom of a slope. However when an area of the concrete and tessera floor was cleared, and the underlying compacted soil surface left exposed overnight, worm casts started to appear. Over the next seven weeks the landowner, TH Farrer, counted the casts on the newly-exposed surface. Farrer also observed burrows through the mortar of the buried walls. Darwin concluded that worms had eventually been able to penetrate the floors when the concrete degraded and cast above them, aiding the process of burial along with the hillwash.

Aided by three of his sons, William, Francis, and Horace, Darwin examined a number of other sites such as Beaulieu Abbey and Silchester in Hampshire, Chedworth Roman villa in Gloucestershire and Wroxeter in Shropshire. At Silchester, he was able to demonstrate how the action of worms could cause the Roman floors to subside more in the middle of rooms than close to the wall, recording several detailed sections.

Darwin's work makes an essential, but often overlooked, contribution to the understanding of how archaeological sites are formed, as well as providing a striking example of early experimental archaeology. Not only did he explain how objects and even buildings left on the surface come to be buried, he also recognised that worms play a role in reducing the height of ancient mounds such as barrows over time. It is fitting that, at a meeting of the British Association for the Advancement of Science held in Dublin in 1958 in celebration of the centenary of Origin of Species, archaeologists – including Richard Atkinson, whose paper entitled "Worms and weathering" had appeared in Antiquity the previous year – established the experimental earthworks committee. Just as Darwin's theories on natural selection and the branching nature of evolutionary progress still form the foundation of biology, so his work on earthworms remains vital to archaeologists interpreting sites.

Down House is open to the public with a new exhibition by English Heritage. All of Darwin's work can be read online. Details of events around the country to mark Darwin's bicentenary can also be found online. Matthew Law is a freelance environmental archaeologist based in Somerset.

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