Tutorial:Dark Earth

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Dark Earth
Dark earth is the name given to urban occupation deposits which are dark coloured and seemingly homogenous. They are common to most cities with long histories, especially those in Europe of Medieval and earlier ancestry. The sediments are usually unstratified, c. 0.5 to 2m thick and contain very few archaeological features. In England, they overlie early and mid-Roman layers in many towns. They consist of dark coloured loam often mixed with building material, pottery fragments, food waste (bone, oyster shells and charred cereal grains), high quantities of phytoliths, and sparse pollen spectra indicating waste ground (the strongly alkaline and oxidising conditions of the sediments causing very poor pollen preservation). This is frequently interpreted as evidence of a sharply reduced urban occupation in late Roman towns, but the earth has destroyed and absorbed late-Roman buildings and made it very difficult to analyse the stratification and date the fourth and fifth centuries.

Geoarchaeological analysis of dark earth
Macphail (1990) describes dark earth at excavations in the centre of Worcester covering Late Roman and Post Roman deposits below identifiable Saxon and Medieval layers. Artefacts useful for dating from the dark earth included an 8th-century coin and pottery from between the mid-3rd century and 9th to 10th century AD. Roman iron working led to high amounts of residual slag occurring in the dark earth, which varies in thicknesses from 200 to 350mm as a uniform deposit spans c. 500 to 600 years.

Micromorphology of the dark earth revealed that the fine fabric material was strongly homogenous, generally non-calcareous and organised into soil structures. These features with many coarse mamillated earthworm excrements and very fine microaggregate Enchytraeid excrements infer total biological reworking and decalcification of lower deposits to form a mull, probable grassland, brown soil. Marked iron and manganese staining at depth and minor gley features in the mull soil indicate it to be a well drained soil only little affected by ground water gleying. Fine, non-woody roots infer the site developed as a grassland mull soil. The presence of birefringent ‘bright rings’, possibly spore cases of vesicular arbuscular mycorrihizae may infer the presence of sheep. All these characteristics indicate that the dark earth was formed by an open grassland field, that had been unaffected by domestic landuse for centuries.

It is likely that biological activity was active throughout the dark earth deposition with soil fauna being brought in on the hooves of animals, whereas grass seeds would have been present in dung and in fodder. Thus the dark earth mull soil probably thickened to its present 200-350mm depth through the biological reworking of a dense trampled deposit, with the addition of on site organic matter from herbaceous growth.