Earthworm Activity and Archaeological Stratigraphy: A Review of

Journal of Archaeological Science (2003) 30, 135–148
doi:10.1006/jasc.2001.0770
Earthworm Activity and Archaeological Stratigraphy:
A Review of Products and Processes
M. G. Canti*
Ancient Monuments Laboratory, Centre for Archaeology, Fort Cumberland, Eastney, PO4 9LD, U.K.
(Received 14 June 2001, revised manuscript accepted 22 October 2001)
The activity of earthworms has long been understood to have significant effects on archaeological stratigraphy, but
many of the details have not been fully grasped by stratigraphers or by the specialists who advise them. Although
worms are widely known to cause the burial of finds due to surface casting, some of their other activities are less
commonly recorded. Amongst these, their active burial of stones and seeds, their building of cairns, and their
production of calcium carbonate granules have mostly not been studied in any detail, and have been little exploited
from an interpretative point of view. This paper reviews the existing knowledge as well as presenting some recent
research carried out into various aspects of worm actions and their relationship to taphonomy.
2002 Elsevier Science Ltd. All rights reserved.
Keywords: EARTHWORM, SITE FORMATION PROCESSES, TAPHONOMY, BIOTURBATION,
GRANULES, BURROWS.
comprehensing their slowness of action. Although we
are mentally well equipped to deal with this problem
today, in Charles Darwin’s time it was harder for
people to see the accumulated effects that he realised
were central to soil formation. Darwin’s (1840) paper
essentially proposed that all stoneless mull topsoils (i.e.
the common topsoils where the humus and mineral
materials are thoroughly mixed) resulted from the
repeated casting of earthworms gradually building up a
fine surface layer. Not everyone believed him. On
reading one of Darwin’s examples, Mr Fish, in the
Gardeners’ Chronicle of 17 April 1869 wrote:
Introduction
owever 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. Why
then, are their effects so poorly understood by almost
all branches of archaeology? Is it from fear of finding
the worst, of finding out just how destructive worm
action is? Or perhaps it stems from the distress that
many feel on seeing their decimation during digging?
Probably the answer lies in the more mundane explanation identified for many years by naturalists and
philosophers—they are simply too small. Gilbert
White wrote in his Natural History and Antiquities of
Selborne:
H
‘‘Considering their weakness, and their size, the work they
are represented to have done is stupendous . . . lifting
4 inches of earth in so short a period . . . staggers one by its
magnitude. . . . This case, like vaulting ambition, carries
one over to the other side, and instead of confirming our
faith in the labour power of worms, tends to destroy it.’’
Darwin was stung enough to use his book on the
subject (Darwin, 1881: 5) to dismiss Fish with the
words:
‘‘The most insignificant insects and reptiles are of much
more consequence, and have much more influence in the
economy of Nature, than the incurious are aware of; and
are mighty in their effect, from their minuteness, which
renders them less an object of attention; and from their
numbers and fecundity. Earth-worms, though in appearance a small and despicable link in the Chain of Nature,
yet, if lost, would make a lamentable chasm.’’
‘‘Here we have an instance of that inability to sum up
the effects of a recurrent cause, which has often retarded
the progress of science, as formerly in the case of geology,
and more recently in the principle of evolution.’’
Perhaps there is a bit of Fish in us all, a tendency to
close our eyes to real processes and hope that sites are
simply fossilized. Much of the mechanics of worm
action was clearly described over forty years ago in
Atkinson’s (1957) paper on the subject, yet, since then,
surprisingly few people have taken on board anything
This unwarranted insignificance, seen by White as
underpinning a general failure to appreciate earthworms’ importance, is compounded by the difficulty of
*E-mail: [email protected]
135
0305–4403/03/$-see front matter
2002 Elsevier Science Ltd. All rights reserved.
136
M. G. Canti
more than the concept that there is some worm disturbance at some sites. We know a little more now than
Atkinson did, but the additional complexity has led
only to further field arguments and more widespread
site myths. This paper attempts to rectify the problem
by describing earthworm processes and products, as
well as exploring the long term results of their range of
activities.
General Biology of Earthworms
There are around 3000 species of earthworm in the
world. The greatest variety is found in tropical soils
with progressively smaller numbers of species further
north. Thus, in France there are about 180 species
(Bouché, 1972) and in the U.K. about 25 (Sims &
Gerard, 1985). Apart from size variations, most species
look physically similar to the untrained eye. Some can
be distinguished in the field, others require microscopic
examination.
Earthworms live either in the leaf litter layer or in
burrows in the mineral soil, and eat almost all forms of
organic matter. They are cold-blooded and breathe
through their skin, both characteristics which make
them vulnerable to sudden environmental change.
Although possessing no eyes, they are light sensitive
through photoreceptors mainly at the head end. This
can be observed by taking a torch on to a lawn on
warm moist evenings after dark, where worms will be
seen lying on the surface with their rear end still in the
ground (see Figure 1). After one or two seconds of
exposure to the light, they rapidly flick back down their
burrows.
All earthworms are hermaphrodite and reproduce
either by mating, self-fertilization or parthenogenesis.
Lumbricus terrestris can sometimes be seen in head-totail mating pairs on the surface, but the majority
of species copulate underground. Earthworm eggcapsules are the 2–5 mm yellowish to yellowish-brown
near-spheres regularly found in topsoil and compost.
Different species interact with different niches in
the same complex ways as other more prominent
animals but in the case of worms, it is particularly
hard for us to see the subtle variations that separate
their underground worlds. A three way grouping of
species and lifestyle is now widely accepted by biologists, based on work by Lee (1959) and Bouché
(1977). The divisions are: litter dwellers (epigeic
species), deep burrowers (anecic species) and horizontal burrowers (endogeic species). The litter
dwellers and horizontal burrowers move around in
temporary holes in the soil itself or on the surface,
while the deep burrowers (anecics) live in more-orless permanent vertical burrows. Not all species fall
immediately into one of the categories, but the system
provides a useful shorthand for ecological tendencies
in morphology and behaviour.
Earthworm Effects on Archaeology
Making burrows
Earthworm burrows are often visible in excavations,
usually where anecic species have been active. The
burrows of species that live near the surface will
typically have been lost during topsoil stripping or are
hardly visible due to the poor colour contrast between
the matrix and the burrow fill. The remains of deeper
burrows show up as numerous near-vertical stripes
about 1–3 cm wide filled with dark topsoil and thus
visible against the lighter coloured subsoil (Figures 1
and 2). The burrow remains open and empty while the
worm is inhabiting it, so this dark fill must represent
either topsoil that has been washed down or the casts
of other worms deliberately placed in the burrow after
its creator has abandoned it. If the burrow is straight,
long portions may be vertically traceable; if it is
sinuous, however, only a few centimetres may show
before it dives back into the section, disappearing from
view. Unused burrows are favoured by plant roots
(Hensen, 1877; Wilcke, 1960; Pitkänen & Nuutinen,
1997) and presumably go through cycles of use and
re-use by both life-forms to the point where they
become indistinguishable.
The permanence of these deep burrows is an important issue archaeologically. Active burrows containing a
live worm may number of the order of 10 per sq. m
(Figure 3) for Lumbricus terrestris (Pitkänen &
Nuutinen, 1997). Clearly, if burrows at these sorts of
densities were being re-dug by the worms regularly,
there would be little prehistoric archaeology left in the
top metre of most temperate climate soils. Many would
be homogenized down to 1·5 m or more (depending on
the depth of burrowing). Since this is not usually the
case, re-use of burrows must be widespread. Let us
imagine a square metre of topsoil (in plan) containing
10 Lumbricus terrestris, each with a lifespan of
5 years, and each homogenizing 1 cm2 of soil per
lifetime. These are deliberately conservative estimates.
Lumbricus terrestris has been known to live for
12 years in laboratory conditions (J. Satchell,
pers. comm.) and the cross-sectional area of homogenized soil in burrows is visibly more than an
average of 1 cm2 in many cases. If a new individual
always digs a fresh burrow for itself through virgin soil
then 10 cm2 of our square metre of topsoil would be
homogenized every 5 years. The whole square metre
would therefore take 5000 years to be homogenized.
Some Neolithic and all earlier material that was not
deeply buried would potentially be affected. This is still
an underestimate as it only applies to mixing by the
population of Lumbricus terrestris. A count of the
remaining earthworm species might well add a further
200–300 individuals per sq. m (see Russell, 1973: 191;
Lee, 1985: 90), some of which are large species such
as Aporrectodea longa and A. caliginosa that also
penetrate deep into the soil and have permanent
burrows (Edwards & Bohlen, 1996). With these new
Earthworm Activity and Archaeological Stratigraphy 137
Leaf-stalks sticking
out of the burrow
Middens built by
Lumbricus terrestris
if stones are present
Lumbricus terrestris
foraging (at night)
Casting
Casts forming
mound around
burrow mouth
Smaller topsoil
species e.g.
Aporrectodea
rosea
Figure 1. Block of soil showing many of the basic features of earthworm lifestyles. A uniform stone/artefact content is depicted (prior to
earthworm sorting—compare with Figure 6).
138
M. G. Canti
Figure 5. Fully homogenized ditch fill at Great Wakering, Essex,
U.K. (Essex County Council Field Archaeology Unit).
Figure 2. Earthworm burrows forming a dense mass of stripes
at Slaughterhouse Lane, Newark, Notts, U.K. (Trent and Peak
Archaeological Trust).
Figure 4. Ditch fill at Great Wakering, Essex, U.K., undergoing
homogenization by worm action (Essex County Council Field
Archaeology Unit).
additions, we might reasonably suggest total homogenization in 3000 years instead, in which case there
would be no Bronze Age or Iron Age stratigraphy in
the top 1·5 m either. The point here is not to argue
Figure 7. Worm-induced lines at Botolph Bridge, Peterborough,
U.K. Note the strong sorting at the bottom of the profile, and a weak
line of stones on their way down in the middle. Modern activity has
scattered another layer on the current surface which will start sinking
rapidly as long as casting continues (Cambridgeshire County Council
Archaeological Field Unit).
whether these figures are precise. They must be broadly
correct, and burrows must, therefore, be re-used over
and over while conditions remain suitable. Only that
way can we explain why wholesale destruction of
prehistoric stratigraphy by the deep burrowers has not
occurred more widely.
Intense worm working can be found occasionally,
providing an opportunity to see this sort of destruction
in progress. On recent excavations at Great Wakering,
Essex, U.K., worm activity was generally widespread,
but had been more effective in some features than
others. Roman and Saxon ditch fills could thus be
seen at different stages of homogenization, having
either partially (Figure 4) or fully (Figure 5) lost their
stratigraphic differentiation.
Where burrows are not re-used for some reasons,
e.g. changes in agricultural practices or water
regimes, the organic matter in the dark fill can act as a
substrate for longer term microbial activity promoting
Earthworm Activity and Archaeological Stratigraphy 139
0
Z (cm)
20
40
60
60
40
80
0
20
40
60
X (cm)
20
80
100
)
m
Y
(c
0
Figure 3. Nine active Lumbricus terrestris burrows in a 10060 cm
study area (from Pitkänen & Nuutinen, 1997).
deposition of iron/manganese compounds in the
burrow. This leads to permanent rust-coloured streaks
in some sections, particularly on coarse textured
substrates such as sand.
Casting and burying finds
Different earthworm have different diets, but most
species ingest combinations of mineral soil and organic
matter in varying proportions. As a result of these
differences, their faeces (casts) are to a greater or lesser
extent bulked out with mineral material. They are
released either in the soil or on the ground surface, and
the choice between these two possible casting positions
has considerable implications for archaeology.
Casting underground mixes the fine matrix of the
soil but tends to leave archaeological layers intact.
Worms simply chew through the soil and, in general,
go around stones or finds. Organic layers will gradually
be destroyed by this process, and we can only guess at
the information that has been lost over the millennia.
However, the large amounts of existing organic
remains testify to the fact that much of what interests
us as archaeologists is in some way outside the worms’
range. Charred plant remains, for example, are presumably unpalatable and pollen-rich sediments tend to
be too anaerobic.
Surface casting is more damaging in so far as it
constantly transports fine material from underground
to the surface. It is this mode of deposition that is
responsible both for burial of finds and for some of the
relative displacement found in stratigraphy. The most
immediately accessible signs that it is going on all
around us are the small piles of soil visible on lawns
and paths in temperate climates. Many excavators also
note numerous casts appearing overnight after stripping or on recently cleaned surfaces. If these casts are
examined closely, they can sometimes be seen to contain elements of dark topsoil and lighter coloured
subsoils, suggesting that they have been brought from
various different layers beneath. There is no definite
depth from which (or even above which) casts are
brought up, but an obvious bias against unnecessary
work and the preponderance of darker coloured castings suggests that the greater part are from nearsurface layers. Any object placed on the surface will
therefore sink rapidly in the first few years of being cast
upon and then gradually slow down. Eventually, it
reaches a depth where no soil destined for surfacecasting is being ingested below it, and there it stops.
Once this point has been reached all subsequent worm
casting activity harmlessly churns the topsoil round
and round above the object, and it will not start
moving downwards again without significant changes
to the soil ecology promoting deeper burrowing
(Figure 6).
Whether different species of worms have significantly different surface-casting capability is less
well understood. The larger soil-dwelling species are
obviously going to be many times more effective (in
terms of soil volume delivered to the surface) than
smaller or surface-living species but actual specific
differences of behaviour are regularly quoted in the
literature. Atkinson (1957) wrote that Aporrectodea
longa* and Aporrectodea caliginosa* (*asterisk denotes
species whose nomenclature has been changed to follow Sims & Gerard, 1985) were the only surface
casting species (meaning in Britain). This appears to
have originated in papers by Evans & Guild (1947) and
Evans (1948), thereafter being re-stated by Atkinson
and various other authors (e.g. Satchell, 1958; Edwards
& Lofty, 1977) with little qualification. However, the
original work was less dogmatic than the subsequent
citation suggests. It described how experience of breeding nine species in pots ‘‘suggested that only [those two]
voided their faeces to any great extent on the surface of
the soil’’ (Evans & Guild, 1947). Field experimentation
backed this view (Evans, 1948) but did not prove it.
Twenty years later, Madge (1969) added Lumbricus
terrestris to the list of surface-casters, as did Russell
(1973: 199). Stein (1983) described this latter species
as casting only on the surface around its burrow opening, while at the same time describing Aporrectodea
caliginosa* as casting below ground. Subsequently,
experimental work has shown that Aporrectodea
rosea casts at the surface at least some of the time
(Thomson & Davies, 1974; Bolton & Phillipson, 1976),
as do Allolobophora chlorotica (Piearce, 1984), Eisenia
andrei, and Aporrectodea giardi (Jégou et al., 1998).
These additional observations call into question the
whole idea of surface and non-surface casting being
species-based. A more realistic approach was hinted at
by Lee (1985: 180) who quoted numerous descriptions
of tropical species that mostly cast underground but
have also been recorded as casting at the surface as
well, implying that some changeable condition must
140
M. G. Canti
Figure 6. The same block of soil as Figure 1, after earthworm sorting. Note that finds and stones have both sunk to a particular depth in
the profile.
stimulate the differing responses. The most likely such
condition would seem to be the degree of compaction.
As the soil bulk density increases, there must come a
point at which worms can no longer cast underground
without blocking their own burrows, so they are forced
to travel to the surface. Darwin (1881: 111) produced a
typically simple and clear approach to this question:
‘‘Worms do not always eject their castings on the surface
of the ground. When they can find any cavity, as when
burrowing in newly turned up earth, or between the stems
Earthworm Activity and Archaeological Stratigraphy 141
of banked up plants, they deposit their castings in such
places. So again any hollow beneath a large stone lying on
the surface is soon filled up with castings.’’
Although very little research has been done on this
subject, what there is now strongly backs the view
that compaction and casting are linked (Thomson &
Davies, 1974; Guild, 1955). A particularly clear case
was provided by Binet & Le Bayon (1999) who found
that a 20% increase in soil compaction led to a doubling
of surface cast production. The idea of surface casting
rates being primarily compaction-driven also seems
intuitively convincing. The link to species would then
be a secondary consideration based on size differences determining the amount of space needed and
thus biasing surface casting rates towards the larger
species.
Why is it significant whether certain species cast at
the surface or not? Partly, it may help us understand
variations in the extent of this important taphonomic
process. Since different species inhabit different niches,
they will affect sites to varying degrees. There is also
the possibility that different species will actually produce different textures in the overlying (upcast) layers
as a result of the maximum particle size that a given
size of worm can ingest. Most studies have put this
value at 2 mm, either presumably or explicitly meaning
for Lumbricus terrestris (Russell, 1957: 113; Kuhnelt,
1976: 121). This cannot be a simple dividing line, but
must rather be a frequency distribution around the
‘‘maximum’’ figure. Canti (1987) achieved a good
match at the coarse end of the particle size spectrum
between measured and calculated size distributions of
upcast and rejected soil at Lismore Fields, Buxton.
This was based on a simple preference model in which
very coarse grains were rejected all of the time and
moderately coarse grains were rejected some of the
time. Refining such a model is tempting, but its applicability to real life situations would depend on knowing the species make up of worms in the soil over the
whole period of casting, due to the wide range of body
sizes exercising the main control over particle size
ingested. Bolton & Phillipson (1976), for example,
measured the largest sand grains in the posterior gut
of Aporrectodea rosea, Aporrectodea caliginosa and
Octolasion cyaneum and found 100 m, 200 m and
500 m respectively reflecting the body size differences
of these three worms.
Broadly similar particle size trends are reflected in
the results of choice experiments (Shumway & Koide,
1994; Schulmann & Tiunov, 1999) and measurements
of cast texture (Zhang & Schrader, 1993; Lal &
Akiremi, 1983; De Vleeschauwer & Lal, 1981). These
types of discrimination would lead to much slower and
subtler changes to stratigraphy than those induced by
the classical sinking of stones. They also all tend to
promote the accumulation of fine material at the
surface, either by direct sorting (Evans, 1948), or by
worm mixing preventing clay translocation from
taking effect (Graham & Wood, 1991).
The quantities of soil brought up by casting vary
enormously. Darwin (1840; 1881) regularly found
depths of 5–6 mm per year being deposited during the
first few years after soil improvement with surface
scatters of fertilizers, but 2–3 mm per year is more
average for temperate Europe (Kevan, 1968: 186; see
also Lee, 1985: 183; Evans & Guild, 1947). Values from
elsewhere on the planet are often higher—e.g. 9–10 mm
per year in New Zealand (Yeates & van der Meulen,
1995) and 15–20 mm per year in Nigeria (Madge,
1969). Viewed the other way round, these figures are
also the rates at which objects initially sink into the
soil. However, the rapid slow down that characterizes
this sinking means that the rate itself is not a particularly valuable figure from the archaeological standpoint. More important is the depth to which objects
finally sink, and the related question of how long it
takes to reach that stability. Various lines of recorded
evidence are useful pointers on these issues. Keith
(1942) re-dug some of Darwin’s sections at Down,
Kent, and found the marker materials to be largely at
the same level as in Darwin’s records—i.e. showing
that the bulk of the worm burial had occurred during
the period of the experiments (the mid 1800s), and the
depth had then remained much the same in the subsequent 80 or so years. Key (1877) found cinders buried
17–18 cm deep on alluvium in Herefordshire, approximately 18 years after they were spread. Canti (1987)
found remnants of lime burning in lines at two depths,
viz. 14–16 cm and 20–23 cm on gritstone head soils in
Buxton, Derbyshire. A single line was also found
15–20 cm down at nearby Harpur Hill. Layers of this
early industrial by-product are common elsewhere in
the Buxton area, where they appear to have been
spread as soil conditioners (Canti, 1993). Although it
seems likely that they are close to the terminal depths
for worm sorted artifacts in the area, the fact of there
being a double line shows the danger of such assumptions. The 14–16 cm (upper) layer presumably still had
about 4 cm to sink, given that the conditions for the
worms are as good now as they were when the lower
line reached its apogee. Lee (1985: 180) stated that
there had to be a terminal depth, because the major
casting species are largely working in the top 10–20 cm,
but he did not attempt top attach an actual figure. It is
also significant that Darwin’s (1881) measurements at
Stonehenge showed only 13–14 cm sinking of the fallen
‘‘Druidical’’ stone—a lot less than many of the modern
examples, despite the long time available for even the
slightest worm action to operate at that site.
It seems that the ultimate depth of artifact burial due
to worm action in normal soils can only be defined to
a fairly broad range, probably 10–25 cm. The time to
stability must be even more elastic. Theoretically one
burrow and one cast per century would gradually
undermine and bury an object, while at the same time,
given worm-friendly conditions, terminal depth can be
reached in as little as 20 years; so we should beware
of overestimating the typical rate based on lines of
142
M. G. Canti
evidence many of which involve soil fertilization processes. Furthermore, dating can be obscured by the
fact that objects from more than one depositional
event on the surface, possibly separated by many years,
can end up in the same buried line (see Figure 7)—a
process well illustrated by Johnson (1990).
Although stone lines can often be seen in stratigraphy, there is a wider range of phenomena that result
from worm action. In any archaeological situation
where there is rich organic matter and enough oxygen,
worms will start working the mineral and organic
materials together and casting wherever they can. The
results can be quite strange, and do not by any means
always conform to a simple model of casting and
objects sinking. For a start, the sinking itself is not
uniform over a distance. Darwin (1881: 194–196)
showed quite clearly that Roman floors of Silchester
were bowed by the effect of greater worm-induced
sinking in the middle of rooms than around the outside, close to the wall. The Reverend J. G. Joyce (then
director of the excavations) tried at first to dispute this
point with Darwin on the grounds that the deeply
founded walls had simply settled less than the shallow
tessellated floor layer. Eventually however, Darwin’s
relentless logic forced Joyce into submission. As
Darwin calmly put it:
‘‘Even Mr. Joyce at last admitted that this could not have
failed to have produced a considerable effect.’’
Constant reworking also inevitably has the effect of
obliterating boundaries. At the Carlston Annis
Mound, Kentucky, U.S.A., Stein (1983) reported that
almost all boundaries had disappeared including burial
pit edges and even the mound/soil interface. Although
worm action does not tend to carry material very far
sideways, the little it does will gradually build up over
a long period to blur lateral distinctions.
The simple fact of worms working around objects
within stratigraphy may also be important. The
example quoted by Barker (1993), illustrates the skin of
dark soil that he often noted surrounding large architectural stones (see his Figure 40). He attributed this to
the worms being forced to go around a large stone on
their way to the surface, commenting on the ease with
which it could be mistaken for a deliberately dug hole.
This is actually quite a difficult point to prove, since a
pit containing an imperfectly-fitting stone would provide the perfect cavity sites for casting by worms, in the
way that Darwin (1881: 111) discussed, and thus
produce a similar morphology. One can easily see the
infilling of such cavities under modern surface stones,
or among the basal gaps in piles of almost any hard
object (e.g. bricks) left on the soil surface for a long
enough time.
Destroying buried soils
The probability that earthworms destroy buried soils
under earthworks was discussed in detail by Atkinson
(1957). Certainly it is common to find no such buried
soil layer, and this is often interpreted as deliberate soil
stripping prior to the monument’s construction. However, as Atkinson (1957) rightly pointed out, cutting
away topsoils before building monuments is a ‘‘curious
and unnecessary thing to do’’, and the action of worms
could well provide a better explanation, particularly
when dealing with obviously secular earthworks.
Once a monument has been constructed on top of a
soil profile, the incumbent worms might find themselves unable to move vertically due to rubble layers
while, at the same time, suffering an unfavourable
gaseous environment if the monument was large
enough to seal off the soil air from the open air above.
These worms would then either move laterally or die,
and the buried topsoil layer would, therefore, remain
intact. If the monument was smaller, however, and
loose enough to allow worms to burrow through it,
they would simply start casting at the new surface.
Soil-eating species would bring up the old topsoil from
below in the same way as they do on gravel drives and
cobbled areas. The result, after many years, would be
the apparent ‘‘truncation’’ of the old ground surface. It
is, unfortunately, very hard to show whether this has
happened. Atkinson, Brailsford & Wakefield (1951)
were able to make a number of deductions about the
phenomenon at the Winterbourne Steepleton pond
barrow, but this was because the surrounding soil
contained large stones whose presence or absence
signifies a great deal in terms of human or worm
actions. Where soils are stoneless, interpretation is far
more problematic.
Taking an overall view, the fact of worms moving
soil up through smaller monuments is not really in
doubt. Evidence for it has been seen on numerous
occasions and is amply backed up by studies such
as Crabtree (1996) showing the exotic spore profile in
the Overton experimental earthwork. However, the
thresholds of monument size and other variables
beyond which the worms can no longer carry out this
action are still largely unknown.
Taking stones and seeds underground
Part of the confusion surrounding the results of earthworm processes has been caused by an incomplete
distinction between their artifact burial through surface casting as described above, and a totally separate
habit in which some species actively take stones down
and deposit them underneath the topsoil (Figure 8).
Darwin (1881: 108) noted the results of this action
at Abinger Roman villa where he found 1–2 mm
stones used to line chambers about 1 m below the
surface. He related these findings to the observation
that glass beads and bits of tile were deliberately
carried down and deposited by captive worms in his
pot experiments. At the time Darwin was working,
W. F. Lindsay Carnegie had noted a similar activity
in Scottish quarries (Burkhardt, 1986: 72–73), while
Earthworm Activity and Archaeological Stratigraphy 143
Figure 8. Abandoned earthworm burrows infilled with topsoil and
fine stones from Narrow Street, London, U.K. (Pre-Construct
Archaeology).
Hensen (1877: 356) and Wilson (1877) recorded it
happening with seeds. Subsequently, although more
detailed work has been done on seed movements (see
Piearce, Roggero & Tipping, 1994), the stone linings
have received little experimental attention, and the
outline facts have only really been restated by various
authors.
In general, it is thought that earthworms line the
terminal chambers as a preparation for quiescent
periods (often called aestivation) when conditions do
not suit them at the surface. Gerard (1967) showed that
the common U.K. species mostly rest up in the
chambers during dry summer weather, the main
exception being Lumbricus terrestris which lives in deep
permanent burrows and is thus unaffected by the
topsoil becoming dry. However, the situation does not
seem to be entirely clear, as Sims & Gerard (1985) later
described the burrow of Lumbricus terrestris as 1–3 m
deep and ending in a stone-lined chamber into which
Lumbricus terrestris does in fact retreat sometimes,
curling up around the stones. Also, how the stonelining benefits the worm during quiescence is unknown.
It has been described as ‘‘keeping conditions suitable’’
or similar phrases by various authors, but there does
not seem to have been any experimental work on the
topic. Darwin speculated that the stones helped to keep
the worms’ bodies away from the cold soil, perhaps
because this would interfere with respiration.
Over time, the chambers collapse, and a visible layer
of tiny stones is sometimes formed, particularly where
floors or other hard layers force the worms to stop
burrowing and make the chambers at a single level.
The material thus produced has been referred to as the
‘‘pea grit’’ and is widely recognized as heralding a floor
or stony layer (Barker, 1993; Limbrey, 1975). It also
sometimes gets deposited at less impenetrable junctions
such as the bottoms of pits, post-holes and shallow
ditches (Atkinson, 1957). Presumably, these latter cases
reflect some conditions (such as moisture-content)
which the worms find more favourable within the fill
rather than just below it.
Although the process does lead to relative finds
displacement, it is confined to very small objects. Bell
(1981: 374–377), for example, found rounded chalk
stones and pieces of mollusc shell in the size range
2–4·7 mm in chambers in his chalk dry valley excavations. Webster (1965) demonstrated that the stones
taken down by worms were typically between 2 and
6 mm in size and can, under some circumstances, build
up to a point where they are visible in nautral soil
profiles, e.g. Long Hanborough, Oxon (Figure 9).
Although this is an extreme case, the generality of the
process can be demonstrated on suitable sites where
stones are available for the worms to drag down. When
compared to the normal matrix soils, burrow fills show
a similar difference of particle size distribution caused
by the worm-selected coarse additions (Figure 10).
Finds of these sizes could lead to false cultural
inferences, but only under rather unusual circumstances. The worms’ predilection for moving seeds (see,
for example, Willems & Huijsmans, 1994) has taken its
toll in this regard. A story was often related by the late
Geoffrey Dimbleby about a feature interpreted as a
granary floor which, on being exposed to light,
sprouted into a mass of seedlings to the embarrassment
of all concerned. This may be an amalgam of a number
of real-life findings, such as the germination that
occurred along the whole length of the worm burrows
at Bedfont (Keepax, 1977).
Making cairns
Another poorly-understood action carried out by some
worm species is their habit of constructing stone and
twig cairns around the entrance to their burrows
(Figure 11). This process has been known to gravel
drive owners for many years but has received little
scientific attention in the literature. The first accounts
seem to have been notes by ‘‘I.W.’’ (1868) and Robert
(1873) drawing attention to the cairns and describing
the authors’ experimental displacement of the stones
followed by their gradual reappearance overnight or
144
M. G. Canti
the extraordinary sight of worms dragging quite
large stones towards their burrows by suction alone
(Figure 12).
The purpose of the cairns is unknown. Various ideas
about protection from predators have been mooted,
but they are not entirely convincing. Large predators
(e.g. badgers) could simply move the stones and small
ones (e.g. the worm-eating slug Testacella) could slide
between them. The stones might, however, offer an
early-warning system against badgers and foxes, both
of which would lose the element of surprise when
ambushing worms lying close to the surface (Trevor
Piearce, pers. comm.). Darwin (1881: 60) speculated
that the cairns were built to prevent cold air from
entering the burrows and maintain humidity—an idea
which might be feasible, but still begs the question
of why the worm cannot simply retreat further down
the hole.
Archaeological records of earthworm cairns have
not been found. Buried surfaces on gravelly soils
should show evidence for cairn-building, but survival
would depend on the manner in which the surface
became preserved. Probably, if they had been seen in
sections at all, they would have been interpreted as
something else (e.g. post pads?).
Dragging smaller finds around on the surface could
be damaging to contextual integrity. Armour-Chelu &
Andrews (1994) found up to 15 cm horizontal displacement of small bones (constrained by their experimental
pots) in a two year experiment. However, over long
periods, they are more likely to be moved back and
forth by the cairn-building process and then eventually
buried by the casts.
Figure 9. Natural profile at Myrtle Farm, Long Hanborough, Oxon,
close to where Webster’s (1965) results were obtained. Note the
central horizon of very small stones thought to represent the long
build up of ‘‘pea grit’’.
over a number of nights. Müller (1889) and Hensen
(1882) described similar structures that were made
mostly of castings with leaf stems sticking out of the
top (see also Figure 1). In fact these are one and the
same phenomenon, the construction materials depending largely on what is available around the burrow. On
closer examination much of the stone cairn’s bulk is
actually formed of castings, with stones sitting on the
top or interspersed amongst them. Each cairn is built
and inhabited by a single worm, and some survive
unchanged for many years (Nuutinen, 1997). Numbers
of cairns have been shown to correlate closely with the
quantity of leaf litter and soil organic matter content
(Nielsen & Hole, 1964), as many as 100 per sq. m being
recorded in suitably rich environments (Subler &
Kirsch, 1998). Although cairn construction occurs at
night, it has actually been observed in progress. The
worm (in this instance Lumbricus terrestris) was
clearly seen to feel around the stone with its mouth,
apparently selecting the ideal spot for grabbing (Davis
et al., 1992). A BBC film has also been made showing
Making granules
As well as drawing small stones down their burrows,
earthworms actually create tiny stones of calcium
carbonate in special glands close to the mouth
(Darwin, 1881; Robertson, 1936). These glands take
the form of two or three pairs of pouches attached to
the oesophagus (Figure 13). The pouches contain white
calcium carbonate either in the form of a paste or as
solid granules which are released into the gut and pass
out with the castings. Although numerous species
possess the glands, it is far from clear to what extent
they are active or what determines that activity. Piearce
(1972) grouped various common species according to
the layout and apparent activity of the glands, showing
that they tended to be most active amongst litter
feeders and inactive amongst mineral soil feeders.
The granules (Figure 14) vary in size from minute
single crystals to stones of over 2 mm (Canti, 1998;
see also Bal, 1977; Ponomareva, 1948) probably depending on species and size of the individual. High
numbers are found in calcareous buried soils and
other Quaternary deposits (Meijer, 1985; Preece, Kemp
& Hutchinson, 1995; Preece, Bridgland & Sharp,
1998), and they may be of considerable value to
Earthworm Activity and Archaeological Stratigraphy 145
Particle size analysis
–3
–2
–1
0
1
2
3
4
5
6
phi
100
90
80
Gt Wakering 2
Percent finer than d
70
60
Gt Wakering 1
50
Narrow Street
40
30
20
10
0
10
20
30
40
50
60
70
80
100
µm
200
300
400
500
600
700
800
1
2
3
4
5
6
7
8
10
mm
Diameter (d)
Figure 10. Particle size distributions of the differences in texture between burrow and matrix soils at Narrow Street, London (Pre-Construct
Archaeology) and Great Wakering, Essex, U.K. (Essex County Council Field Archaeology Unit). These diagrams have been created by
measuring the burrow fill particle size distribution and subtracting the matrix soil particle size distribution in each case. This highlights the
similarity of the size range actually taken down by the worms.
Figure 11. Cairn of stones and twigs around a burrow mouth.
palaeoenvironmental interpretation. However, the
complex interplay of taphonomic and physiological
factors controlling granule concentration in the soil has
yet to be unravelled. Experiments are currently under-
way to help elucidate these issues, but long-term data is
needed before the value can be fully exploited. For
example, the depth at which the granules are deposited
is barely known in any detail. Although some granules
are found in the subsoil, most occur in the uppermost
layers presumably reflecting a tendency towards
surface cast deposition. Preliminary results lending
credence to this suggestion are presented in Figure 15.
Mixed unidentified worms were kept in three sieved
soils of known composition to determine rates of
granule production under different conditions. The full
results will be published in future when the experiments
are complete, but the preliminary granule counts are
already providing interesting information. When the
wormeries were broken up, the number of granules
found at different depths were counted. In each case,
regardless of the experimental conditions, the largest
granule concentration always occurred at the top of the
profile and there was a general progressive decrease
occurring with depth. This abundance pattern could be
very useful for determining the position of buried
146
M. G. Canti
Figure 14. Modern earthworm granules produced by Lumbricus
terrestris under experimental conditions.
Figure 12. A still shot from the BBC video ‘‘Nature Detectives’’
showing a worm dragging a stone towards its burrow mouth (BBC
Bristol).
Granules per 50 g soil
0
Mouth
Oesophagus
Oesophagus
Oesophagal
pouch
Concretion
Soil depth (cm)
Pharynx
IX
X
XI
Calciferous glands
Oesophagal
glands
Oesophagus
XII
XIII
XIV
Crop
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Figure 15. Numbers of granules produced at different depths in
three experimental wormeries.
Gizzard
Upper part of intestine
Figure 13. The calciferous glands of Lumbricus terrestris from
original drawings by Darwin (1881) and Robertson (1936).
land surfaces in complex stratigraphy or where other
indicators have disappeared.
Various other effects need also to be taken into
account when considering granule numbers in soils.
Earthworms are consumed in prodigious quantities by
numerous animals, particularly the badger (Meles
meles). Both recycling and destruction of granules will
thus be occurring via predator digestion and defecation
(see Canti, 1998). At the same time, depending on soil
pH, many granules must be lost by simple dissolution.
Summary
Six major headings encompassing the range of earthworm products and actions have been discussed. These
can be summarized as follows:
(1) Making burrows
Numerous temporary and permanent burroiws exist in
the topsoil. The dark vertical streaks representing
permanent burrows are only visible in subsoils due
to colour contrast. In northern Europe, these deeper
burrows mostly belong to Lumbricus terrestris.
Earthworm Activity and Archaeological Stratigraphy 147
(2) Casting and burying finds
Worms can bring up 1–10 mm per year of soil and
deposit it on to the surface. Since they collect it from
the whole soil profile, surface stones and finds sink into
a line. Faint or strong traces of stone lines mark the
base of old topsoils. These can be found preserved
throughout sections as long as soil has accreted (e.g.
through colluviation) and has not been worm-worked
again.
(3) Destroying buried soils
Shallowly-buried soils are definitely destroyed by
worms. They bring the old topsoil up through the
overburden and deposit it on the new surface. A
similar process seems likely in some other situations
where monuments lie on a truncated surface, but is is
hard to prove.
(4) Taking stones and seeds underground
Small stones, seeds and sand grains in the size range
5 mm–200 m are actively transported down into the
burrows. Some are used to line aestivation chambers
and end up as the ‘‘pea grit’’ layer after abandonment.
(5) Making cairns
Gravel and small stones are transported across the
surface towards the burrow entrance and accumulate
in a conical pile. If stones are not present at the surface,
then twigs are drawn to the burrow as well as rubbish,
finds etc. The cairns have not yet been reported
archaeologically but must be present in some
situations.
(6) Making granules
Granules of calcium carbonate are released into the gut
from special glands near the mouth in some species.
These can be found in large numbers in buried soils
and throughout archaeological sections such as ditch
fills. A fuller understanding of their function and
taphonomy is required to determine their interpretative
value.
The results of earthworm activity need to be fully
grasped by archaeologists as an integral component of
the earth surface processes that affect site interpretation. This article has presented both the historical
perspective and the most recently available knowledge of the major actions and products of earthworms. It is hoped that it will lead to a greater
confidence amongst stratigraphers about the effects
that can be correctly ascribed to these extraordinary
yet ubiquitous creatures.
Acknowledgements
Thanks to Kerstin Eckstein (Institut fur Metalkunde,
Freiberg), Olga Khoklova (Russian Academy of
Sciences, Moscow), Visa Nuutinen (Agricultural
Research Centre of Finland), Trevor Piearce
(University of Lancaster), Stefan Schrader (Zoologisches Institut der TU Braunschweig) and Jean-Luc
Schwenninger (Royal Holloway, Egham, U.K.) for
references and translations.
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