Dark earth in archaeology is an archaeological horizon (sometimes somewhat incorrectly called layer or strata), often as much as 0.6 m – 0.9 m (2 – 3 ft) thick, indicating settlement and urbanized areas during longer time periods.
In England, dark earth covers Roman remains, notably in London and in Roman ruins in the rest of England, particularly urban ones. The stratum underlying the dark earth is often of a date varying from the 2nd to the 5th century AD, and the stratum overlying is often, in the City of London, 9th century. The Dark Earth shows little evidence of any depositional structure in it or even of horizons, although tip lines are sometimes seen.
In Sweden, dark earth of 40 hectares has been found in Uppåkra (in southernmost Sweden, former Denmark), where city-like settlement existed from about the year of 0 until 1000 C.E.[clarification needed] when the settlement was moved to today's city of Lund. Dark earth of 7 hectares has been found in the Viking city of Björkö (today called Birka), in central Sweden, close to today's Stockholm. Dark earth has also been found in Köpingsvik, at the island of Öland close to the southern Sweden east cost.
The material is high in organic matter, including charcoal, which gives it the characteristic dark colour; it also contains fragments of brick and tile. It may represent vacant lots on the edge of urban centres. In London, it has been taken as evidence of the decline of Londinium's population or of its partial displacement outside the city walls.
Recent discoveries, circa 2004, have changed scientific opinion on dark earth: "People thought the [Roman] empire fell and the cities turned into garden [plots]. That is how dark earth was understood up until about five years ago. ... In the Roman excavations there were pots and stone buildings and columns." On top of this is a layer of dark, humus-looking soil, the dark earth. This layer is believed to have been caused by the shift from stone to organic building materials, as well as the breakdown of Roman city sanitation and garbage removal. "They had thatched roofs and wooden houses, they didn't have Roman garbage removal, and they just dumped the ashes and charcoal from their hearths out in the road and all of that compacted."
Some archaeologists see dark earth as reworked urban stratigraphy, indicating timber, smoke-impregnated thatch, decayed weeds, and earth floors reworked by worm action. They argue that late Roman cemeteries around London do not show a population decline compared with earlier London. More recent "reworked stratigraphy" ideas are based on theories that abandoned soils were reworked by agricultural action, such as ploughing, which mixed building materials from the abandoned Roman cities into stratigraphy higher up the sequence.
- Michael McCormick, quoted in "Who Killed the Men of England?", by Jonathan Shaw, Harvard Magazine, July-August 2009
- Macphail, Richard I.; Galinié, Henri; Verhaeghe, Frans (2003). "A future for Dark Earth?". Antiquity 77 (296): 349–358.
- Darwin on earthworms