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Radiocarbon results are not calendar dates read straight from a bone or layer of silt. They measure remaining radioactive carbon and must be calibrated against a curve built from independently dated environmental archives. Beyond 30,000 years ago, the relation between radiocarbon years and calendar years is especially uneven, so errors and ranges matter greatly. The important local chronological anchor is a freshwater deposit exposed at Derryvree, near Maguiresbridge in County Fermanagh. Organic silts and fine sands trapped between two sheets of glacial till produced a radiocarbon determination of 30,500+1170−1030 radiocarbon years BP. With modern calibration, that determination spans broadly about 36,900–31,700 cal BP. Thus 31,769 BCE lies close to the younger edge of a dated environmental window, rather than at a point archaeology can isolate.
The principal anchor is a road-cut through a drumlin at Derryvree, near Maguiresbridge in County Fermanagh. Excavations in 1969 revealed organic freshwater silts and fine sands enclosed between two thick, distinct sheets of till: unsorted debris deposited by glacier ice. The sequence gives a clear relative order. An earlier glacier had laid down the lower till; a freshwater environment then developed; and a later advance buried the deposits beneath another till. The organic horizon produced the conventional radiocarbon measurement Birm-166, 30,500 years BP, with a substantial uncertainty of minus 1,030 and plus 1,170 years.
Radiocarbon laboratories conventionally report age in years before AD 1950, and a radiocarbon determination is not automatically a calendar age. Atmospheric carbon levels varied through time, so measurements must be calibrated against independently dated records. IntCal20, the current Northern Hemisphere calibration curve extending to 55,000 calendar years before present, has improved this process but has not made the far past precise to individual years. Thus 31,797 BCE is best treated as approximately 33,746 calendar years before AD 1950, not as a date at which a particular Irish occurrence happened. Where published scholarship provides ranges, those ranges matter more than a calculated BCE label. The Last Glacial Maximum, for example, is generally placed at about 28,200–20,400 calibrated years before present, while the date considered here belongs to the preceding interval.
Marine Isotope Stage 3, conventionally dated to about 57,000–29,000 years ago, was neither a single uniform winter nor a settled climatic age. North Atlantic temperatures shifted repeatedly between colder stadials and relatively milder interstadials. Those changes affected the reach of glaciers, the position of coasts, vegetation and animal movements. A notional date around 31,806 BCE falls late in this phase, several thousand years before the fullest expansion of the last British–Irish Ice Sheet, generally placed around 27,000–23,000 calibrated years before present.
