The third and fourth centuries at London are characterized by the widespread presence of dark humic soil, sometimes more than a yard thick and with cultural debris (pottery, bones of butchered animals, glass fragments) mixed into it, covering occupational remains of earlier centuries. This material, known as dark earth, is not unique to London but has been identified at many urban sites all over Northern Europe during late Roman times. There has been a great deal of discussion and debate about this dark earth--what it represents and how it originated. It was once widely regarded as evidence of decline and abandonment of Roman urban centers. It was considered a natural soil that developed on top of areas that had once been active parts of the urban center, such as we might observe forming in a vacant lot in a city today.Peter S. Wells, Barbarians To Angels: The Dark Ages Reconsidered. pp. 111-113
For a variety of reasons, this interpretation has changed. the dark earth is not thought to represent not abandonment but rather thriving activity--but activity of a very different character from that of the Roman urban centers. The dark earth has been found to contain remains of timber-framed, wattle-and-daub huts, along with sherds of pottery and metal ornaments datable to the late Roman period. These observations demonstrate that people who were living on the site were building their houses in the traditional British style rather than in the stone and cement fashion of elite and public Roman architecture. Such structures are much more difficult to identify in the archaeological material than Roman buildings, because all that is typically left of them are postholes in the ground and crumbly fragments of the daub that had been packed around the branchwork of the walls.
What are we to make of these two major changes reflected in the archaeology? After rapid growth in the latter part of the first century, London emerged as a stunning center of the Roman Empire on its northern edge, with the monumental architecture, a thriving commericial center, and a military base characteristic of the greatest of Roman cities. The third and fourth centuries at London are marked by a stoppage in major public architecture and a reverse of that process, the dismantling of major stone monuments, at the same time that much of the formerly urban area seems to have reverted to a non-urban character.
To call these changes "decline," "collapse," or "abandonment"--as has been done in the past--is to adopt a conservative Roman attitude toward change. Because we live in societies that use monumental stone architecture in ways similar to how Rome used it, we tend to think of dismantling such structures--our monuments to military and civic glory--as distasteful. But the question we need to ask is, how can we understand these changes in terms of the lives and actions of the inhabitants of this specific place?
All of these changes--the dismantling of stone buildings, the reuse of stone, and the buildup of the dark earth--can be understood in terms of new uses of the formerly urban landscape for different purposes. As evidence accumulates in London, it is becoming clear that the site was not abandoned, as earlier researchers had thought. Life went on in place throughout the third, fourth, and fifth centuries; it was just different. For reasons that are explored below, the inhabitants of London after the glory years of the second century did not have uses for the monumental stone structures that played such important roles between A.D. 70 and 200. Their needs were different, and they behaved in ways dictated by their traditions and their uses of the resources available to them.
This fascinating discovery seems to be underreported. Even over a period of centuries, the building of that much organic soil is quite a significant achievement. In nature, topsoil formation is extremely slow, in fact it takes a "natural" system hundreds of years to produce about an inch of topsoil. So deposits of humic soils up to three feet deep are quite a find! Are these anthropogenic (man-made) soils? If so, how was it done? And why were these found in cities? Were there urban gardens? Urban farming? Livestock grazing? Composting? Were early Europeans practicing biochar (carbon farming)? Were they practicing an early form of Hugelkutur? If so, why is it only found during the time period of the Dark Ages? Written sources don't exist, of course, but further scientific study could be instructive.
One is reminded of the recent discovery of Terra Preta soils in the Amazon basin:
Amazonian Dark Earths (ADE), one of the best-known examples of anthropogenic (man-made) soils, are the result of Amerindian settlements in the pre-Columbian period. ADE are highly variable in terms of their size, shape, depth and physical and chemical make-up. Scholars tend to divide ADE into two categories: terra preta and terra mulata. The former are dark and highly fertile soils replete with ceramic shards, indicating former areas of habitation, while the latter are lighter in colour, less fertile, lacking pottery and thought to be old agricultural fields. While a scientific consensus on the origins of terra preta has existed for several decades, the origins of terra mulata remain enigmatic and contested.
I had no idea until I read the book cited above that similar things were found in European settlements during the Dark Ages. One of the major advances in recent historical understanding is the role that topsoil depletion has played in the decline and fall of past civilizations, including those of ancient Mesopotamia, the Roman Empire and the Lowland Classic Maya. Wikipedia says:
Topsoil depletion occurs when the nutrient-rich organic topsoil, which takes hundreds to thousands of years to build up under natural conditions, is eroded or depleted of its original organic material. Historically, many past civilizations' collapses can be attributed to the depletion of the topsoil. Since the beginning of agricultural production in the Great Plains of North America in the 1880s, about one-half of its topsoil has disappeared. Depletion may occur through a variety of other effects, including overtillage (which damages soil structure), overuse of inputs such as synthetic fertilizers and herbicides (which leave residues and buildups that inhibit microorganisms), and salinization of soil.
Topsoil depletion is a significant issue throughout the world. The message is clear: destroy healthy, productive soil and destroy civilization. The question is, can it be done without the collapse of large-scale industrial agriculture? The late Roman period was characterized by large landed estates called latifundia; essentially large plantations which were worked by slaves producing export crops. Wikipedia calls them the closest thing the ancient world had to modern industrial agriculture. Once Roman military and legal control slowly faded away, latifundia declined as well, since long-distance shipping of commodities like grain and olive oil declined and food production was retooled for local production. Slaves disappeared, and farming was once again done on smaller scales by free farmers (although most were serfs). With so much land in the hands of industrial farming, can we transition to more benign methods without a collapse? What do these dark earths have to teach us?
Taking this out of the past and into today, there is a case in our own time similar to the collapse of the Roman Empire's trade networks in Northern Europe. When the Soviet Union collapsed, Cuba was isolated from it's main trading partner on the other side of the Atlantic, and surrounded by hostile states. The ships from Russia stopped coming. The situation in Cuba must have been very similar to that faced in sub-Roman Britain. Without the trading networks, Roman estates geared for export crops would have foundered. Here is what happened in Cuba, according to Wikiepdia articles of the event:
During the Cold War, the Cuban economy relied heavily on support from the Soviet Union. In exchange for sugar, Cuba received subsidized oil, chemical fertilizers, pesticides and other farm products. Approximately 50 percent of Cuba's food was imported. Cuba's food production was organized around Soviet-style, large-scale, industrial agricultural collectives. Before the collapse of the Soviet Union, Cuba used more than 1 million tons of synthetic fertilizers a year and up to 35,000 tons of herbicides and pesticides a year.
With the USSR collapsed, Cuba lost its main trading partner and the favorable trade subsidies it received, as well as access to oil, chemical fertilizers, pesticides etc. From 1989 to 1993, the Cuban economy contracted by 35 percent; foreign trade dropped 75 percent. Without Soviet aid, domestic agriculture production fell by half. This time, called in Cuba the Special Period, food scarcities became acute. The average per capita calorie intake fell from 2,900 a day in 1989 to 1,800 calories in 1995. Protein consumption plummeted 40 percent.
Without food, Cubans had to learn to start growing their own food rather than importing it. This was done through small private farms and thousands of pocket-sized urban market gardens—and, lacking chemicals and fertilizers, food became de facto organic. Thousands of new urban individual farmers called parceleros (for their parcelos, or plots) emerged. They formed and developed farmer cooperatives and farmers markets. These urban farmers found the support of the Cuban Ministry of Agriculture (MINAGRI), who provided university experts to train volunteers with organic pesticides and beneficial insects. These efforts were furthered by Australian agriculturalists that came to the island in 1993 to teach permaculture, a sustainable agricultural system, and to "train the trainers". The Cuban government then sent these teams throughout the country to train others.
Due to a poor economy, there were many crumbling buildings that could not be repaired. These were torn down and the empty lots lay idle for years until the food shortages forced Cuban citizens to make use of every piece of land. Initially, this was an ad-hoc process where ordinary Cubans took the initiative to grow their own food in whatever piece of land was available. The government encouraged this practice and later assisted in promoting it. Urban gardens sprung up throughout the capital of Havana and other urban centers on roof-tops, patios, and unused parking lots in raised beds as well as "squatting" on empty lots. In Havana, organopónicos (organic farms and gardens) were created in vacant lots, old parking lots, abandoned building sites and even spaces between roads.
Cuba's history of colonization included deforestation and overuse of its agricultural land. Before the crisis, Cuba used more pesticides than the U.S.. Much of their land was so damaged (de-mineralized and almost sand-like) that it took three to five years of intensely "healing" the soil with amendments, compost, "green manure", and practices such as crop rotation and inter-planting (mixed crops grown in same plot) to return it to a healthy state. Bio-fertilizers and bio-pesticides have replaced most chemicals. Today, 80% of Cuba's produce is organically grown. Without the fertilizers, hydroponic units from the Soviet Union were no longer usable. The systems were then converted for the use of organic gardening. The original hydroponic units, long cement planting troughs and raised metal containers, were filled with composted sugar waste and hydroponicos became organopónicos.
Another reason Cuba survived this crisis is the shift in their thinking from machine to manual labour. Abandoning their previous industrialized agricultural methods, tractors and other machinery were replaced with human and animal labor. Older farmers familiar with raising and training oxen trained others to increase those involved in food production. Chemical fertilizers were replaced with organic farming techniques which require more labor but less fossil fuels.
Cuba has more than 7,000 organopónicos. More than 200 gardens in Havana supply its citizens with more than 90% of their fruit and vegetables. Yields have more than quintupled from 4 to 24 kilograms per meter squared between 1994 and 1999, and currently around a million tons of food per year is produced in the organopónicos. More than 35,000 hectares (over 87,000 acres) of land are being used in urban agriculture in Havana alone. The city of Havana produces enough food for each resident to receive a daily serving of 280 grams (9.88 ounces) of fruits and vegetables. The urban agricultural workforce in Havana has grown from 9,000 in 1999 to 23,000 in 2001 to more than 44,000 in 2006. "Kiosks" (farmers' markets) were set up in all communities to provide easy access to locally grown produce; less travel time required less energy use. These local markets provide 80-100% of the produce needed for that local community.
One can imagine some similarities between this and what happened to far-flung provinces in the Roman Empire, where stone architecture was dismantled, and food production was moved much closer to the cities. Perhaps this explains some of the mystery of the Dark Earth.
The overall message is a hopeful one. Wells' passage cited above suggests that societal change can be transformational without being destructive, and that human actions done rightly can preserve and even enhance the environment, while producing abundant food.
For a modern take on rapid topsoil formation, please see this terrific article by Permaculturalist Ben Falk: Inheriting Subsoil, Leaving Loam.