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The Derryvree organic deposit produced the conventional radiocarbon determination BIRM-166, reported as 30,500 radiocarbon years BP with a substantial uncertainty of +1,170/−1,030 years. A radiocarbon age is not equivalent to a calendar age. Atmospheric carbon-14 concentrations changed through time, so laboratory measurements must be compared with calibration curves such as IntCal20. At this antiquity, calibration produces broad probability distributions rather than a precise date. The deposit is generally placed in the middle part of the Midlandian, the Irish name often used for the last glacial stage.
Derryvree was exposed during roadworks in 1969, when a cutting passed through a drumlin: a streamlined hill of glacial sediment. Beneath its surface were two distinct tills, unsorted deposits laid down by ice. Between them lay freshwater silts, fine sands and organic-rich mud. This sequence supplies the firmest conclusion available. Glacial ice had occupied the locality and left the lower till; at a later point, water and vegetation accumulated in a basin not overridden by ice; eventually another ice advance buried the deposit beneath the upper till.
Derryvree records a local interval in which freshwater accumulated after one phase of glaciation and before later ice or glacial deposits sealed the site. This sequence is important precisely because later glaciers removed, rearranged or buried so much of Ireland’s older landscape. It offers unusually direct evidence that part of Fermanagh was not continuously beneath ice during this portion of the Midlandian glacial cycle. Yet it must not be inflated into a map of the entire island. A small wetland basin in south Fermanagh cannot establish the precise state of every coast, mountain or river valley in Ireland.
The distinction between a radiocarbon age and a calendar age matters especially here. Radiocarbon years are calculated from the remaining carbon-14 in once-living material; they are not identical to calendar years. They must be calibrated against a curve that models changing atmospheric carbon-14 levels. IntCal20 extends such calibration to 55,000 calendar years before present, but its older sections draw on several natural archives and carry meaningful uncertainty. A date expressed as 31,752 BCE may be useful for arranging an archive, but it must not imply that Ireland’s climate, coastlines or inhabitants can be described with annual precision.
