Dark earth

Dark earth in archaeology is an archaeological horizon, as much as 1 m (2 – 3 ft) thick, indicating settlement over long periods of time. The material is high in organic matter, including charcoal, which gives it its characteristic dark colour; it may also contain fragments of pottery, tile, animal bone and other artefacts. It is interpreted as soil enriched with the sooty remains of thatched roofs from houses without chimneys, with other waste materials. In some areas it appears to give the soil added fertility.

London's dark earth was originally called 'black earth' by archaeologists. It was renamed dark earth because of confusion with the chernozem (black earth soils in Russia; in these, the dark colour is traditionally (not universally) thought to come from humus, rather than soot).

In the Hebrides, it was customary to remove the thatch from the "black houses" every spring, and spread it on the fields as fertilizer, improved by the soot which it retained. On Achill Island, special smoke huts were built in the fields, stone structures with sod roofs. From October to May smoky fires burned inside them, and in spring the sods were spread on the fields. In the Amazon basin, there are very extensive areas of dark soil, known as terra preta, enriched by small particles of soot, and these areas are much more fertile than the natural soil.

In England, dark earth covers many areas that were built up in the Roman period, especially Londinium. In some cases, it may represent open spaces on the edge of urban centres, but can also be found in more rural settings in and around foci of settlement. In the example of London, deposits underlying the ancient city's dark earth are often dated to between the 2nd to 5th century, the middle and later Roman period. Overlying deposits are frequently dated to the 9th century when Saxon London was repopulated and began to expand. The dark earth shows little evidence of any depositional structure or 'horizons', although tip lines are sometimes recorded.

Archaeologists have debated what London's dark earth layer may indicate about human use of the city. It has been taken as evidence of refuse disposal or gardening during the Roman period on the site of previous buildings. In this case it might be evidence of a decline of Londinium's population, or of its partial displacement outside the city walls. However, late Roman cemeteries around London do not show a population decline compared with earlier London.

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