Fresh Earth on a Roman Floor

When the earth was cleared from a Roman floor at Abinger in Surrey in August 1877, Charles Darwin initially doubted that worms could explain what had covered it. The concrete seemed sound. Around it stood the remains of thick walls, still joined to the floor. Perhaps the fine earth above the ruins had simply washed down the sloping field.

By the next morning, little cakes of earth had been lifted over seven burrows. On the third morning, twenty-five openings were counted. When some of the cakes were suddenly raised, four worms retreated into the ground. Two substantial heaps of worm-cast earth had appeared during the third night.

The excavation had exposed a floor on which earth was still being deposited. Darwin needed to know how much work those small inhabitants could do, and whether their present activity could help explain the disappearance of a building over centuries.

A surface that would not stay put

Darwin was already famous for his explanation of evolution by natural selection. His earthworm investigation had begun much earlier than his visit to Abinger, and eventually became The Formation of Vegetable Mould, through the Action of Worms, published in 1881. The title’s “vegetable mould” meant the dark earth beneath grass and other vegetation. He was studying how that layer formed and moved.

His uncle Josiah Wedgwood had suggested the explanation that set him working. Around Maer Hall in Staffordshire, England, materials spread over pasture could later be found beneath it. White lime and dark cinders remained recognisable as buried layers. Worms were bringing fine earth up and depositing it on the surface, gradually covering what had once lain there.

A casting is material passed through a worm’s gut and left behind. Rain can disperse the fresh coils; dry casts can crumble. A sequence of small, recognisable deposits can become a more uniform covering. The pasture’s apparent stillness conceals movement between the ground beneath it and its surface.

Darwin presented a paper on the formation of mould to the Geological Society of London in 1837. He continued to use conspicuous fragments as markers. On 20 December 1842, broken chalk was spread on old pasture near his home at Down in Kent. When a trench was dug in November 1871, the white nodules formed a line seven inches below the surface. He had allowed the field twenty-nine years to answer.

There was a familiar example closer to the house. A narrow path had been paved with small stones in 1843. Worm castings and weeds appeared between them. The gardener swept and weeded it for several years, then gave up sweeping and mowed it with the lawn. The path disappeared. When its turf was removed in 1877, the stones were still in position, beneath an inch of fine earth.

The garden supplied long experiments alongside more immediate ones. Darwin kept worms in earth-filled pots in his study and tested how they responded to light, touch and vibration. They remained quiet beside a loudly played piano. Pots placed on the instrument produced a different result: when notes were struck, the worms withdrew. Vibrations could reach them through the saucer, pot and earth.

The distinction between sound travelling through air and vibration through a solid object mattered to the experiment. Darwin was trying to identify what reached the animal and caused a response. The apparently uneventful pot beside the piano was part of the result.

Down House combined a family garden, a place to think and spaces for experimental work. Its present-day appearance also involves reconstruction from the surviving records. In English Heritage’s film, head gardener Rowan Blaik explains how letters, pictures and notebooks guide the treatment of the grounds. This is the maintained garden today, rather than a surviving film of Darwin’s experiments.

A closer look · 3:24 · English

A garden used for experiments

Play film

Collecting enough to weigh

Buried markers showed the result of years of activity. A second approach could measure the material arriving at the surface. Near Leith Hill Place in Surrey, a woman whom Darwin did not name in the book undertook to collect castings from two plots, each one square yard.

One plot lay on an old grassy terrace. It had a shallow covering of dark earth above sandy ground and sandstone fragments, and appeared comparatively unfavourable for worms. The other lay on common land near Leith Hill Tower, under short turf. Existing castings were cleared before collection began, so that the old deposits would not enter the new total.

The task lasted from autumn 1870 into autumn 1871. It could not recover every particle. Rain washed fine earth away, grass held onto pieces, and dry sandy casts fell apart. The collector sometimes left home for a week or two. The more distant common was visited less frequently than the terrace. Darwin described these losses before giving the weights.

After drying, the common’s collection weighed 7.453 pounds. It represented 367 days’ work on one square yard. An acre contains 4,840 square yards. If an acre of comparable land yielded at the same rate, multiplying 7.453 by 4,840 gave a little over 36,000 pounds. Dividing by the 2,240 pounds in a British ton gave Darwin’s figure of about 16.1 tons for an acre over roughly a year.

That calculation carried a condition: the larger area had to behave like the sampled plot. The terrace produced a different figure. Its collection weighed three and a half pounds after 369 days, equivalent to about 7.56 tons on an acre of similar ground. Neither square yard established a rate for every field.

The quantities measured earth being handled, much of it repeatedly. They did not mean that sixteen tons of entirely new soil had been created. Worms could swallow fine material already near the surface, bring it up, and later encounter it again. Their digging also left passages that could collapse. Objects could acquire more earth above them while losing support beneath.

A knitting needle on the hillside

Darwin’s measurements depended on other observers as well as long waits. His son William examined fallen stones at Stonehenge with a trowel and skewer. William’s letter distinguished uncertain observations from those he thought useful: some stones were undermined by rats or rabbits, and a shepherd thought the ground beside one block might have been disturbed. The burial of a fallen stone was evidence to investigate, rather than a ready-made clock.

Darwin also wanted to know the direction of worm burrows on steep grassy slopes. Down lacked a suitable slope for this particular test. On 5 January 1872 he asked his niece Lucy Caroline Wedgwood to try probing holes with a straight, blunt knitting needle. He hoped that burrows emerging at right angles to the slope would help explain the downhill transfer of earth.

Lucy reported on twenty-five holes. Eight came to the surface “nearly vertical to the slope”; the rest ran at various angles. Darwin thanked her, admitting that prolonged stooping tired his head. Even the word “vertical” needed clarification. His mathematician son George pointed out that it referred to the horizon; did Lucy mean perpendicular to the slope? Darwin was still unsure whether he could form a judgement.

Seven weeks on a Roman floor

At Abinger, Thomas Henry Farrer could continue watching after Darwin’s visit. Farrer had extended the search after ancient remains were found in a farmyard in 1876. The work revealed rooms, masonry and a concrete floor still partly covered by small tiles.

The apparently solid surface was uneven in its resistance to worms. They could pass through softer concrete and gaps between the tiles. Some solid patches remained unpenetrated and held rainwater. Burrows also entered decayed mortar in the surrounding walls. Fresh earth arriving above ground had a route through the ruins.

Farrer recorded the activity over the following seven weeks. He flattened signs of activity so that reopened holes and new castings could be recognised. On 26 August, after heavy rain had cleaned the surface, forty openings were counted. By 13 October there were sixty-one. Openings and castings were recorded separately: an old burrow could reopen without its occupant leaving a heap of earth.

Darwin inferred that work beneath the floor, repeated over long periods, had helped undermine it, while castings accumulated above. He also allowed for earth washed down the field, dust and other causes. Demolition rubble in a city was a different problem from fine earth over a deserted country building. Worms did not have to explain the entire deposit.

Three years later, at Darwin’s request, Farrer looked at the concrete floor again. The worms were still working.