Prehistoric earthworks at Bosigran, in west Cornwall, have been directly dated for the first time, shedding new light on the area’s ancient field systems.
Acquired by the National Trust in 1978, Bosigran is home to granite-and-earth boundaries forming small fields, dubbed the ‘work of giants’ and long believed to be prehistoric in origin. A detailed survey carried out in 1981-1983 established relative chronologies that identified six distinct types of field pattern attributable to periods from the 2nd millennium BC to the post-medieval period. However, establishing accurate dates for their creation and development remained a challenge.
There are few ancient field systems around the world that have been directly dated, as many of the existing approaches – which include dating artefacts recovered from earthworks, dating through associated archaeological features, and direct dating of ecofacts and sediments using radiocarbon or optically stimulated luminescence (OSL) methods – can be problematic.
In this new study, published in Antiquity (https://doi.org/10.15184/aqy.2019.138), conventional OSL dating in the laboratory was combined with practical field-profiling methods. Small trenches were cut in selected boundary banks to collect soil samples along the entire sediment stratigraphy, and a portable OSL reader used to measure the luminescence signals of the samples in the field. This immediately provided a relative chronology for the earthworks’ development, which informed the strategy for larger dating samples. Once laboratory analyses were complete, it was possible to create highly detailed accounts of the earthworks’ history.
OSL profiling and dating (OSL-PD) confirmed the findings of the 1980s survey: that some earthworks date to the middle Bronze Age (2nd millennium BC) and middle Iron Age (400-100 BC). It also revealed, however, that the Iron Age boundaries, which were assumed to have been fairly substantial by c.AD 500, had in fact remained relatively low until that time, suggesting that the landscape was exploited in the same way from the middle Iron Age up to the early medieval period, when a change in management practices occurred.
The study has proved the prehistoric origins of Bosigran’s fields and changed understandings of the evolution of the landscape over time. It also demonstrates that the OSL-PD method is an effective and practical way of dating earthworks, which could be applied to other sites in the future.