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Derryvree was exposed in a road cutting in 1969. The section revealed freshwater silts and fine sands, including organic-rich layers, sealed between two thick sheets of glacial till. This stratigraphy matters. The lower till records an earlier episode when ice had overridden the locality; the organic sediments indicate that the ground was later free of glacier ice long enough for water, plants, insects and sediment to accumulate; and the upper till shows that ice subsequently returned. The site is not a snapshot of all Ireland. It is, however, compelling local evidence that part of Fermanagh could be ice-free during a cold phase before the greatest expansion of the last British–Irish Ice Sheet.
This was part of Marine Isotope Stage 3 (MIS 3), a long, climatically unstable phase of the last glacial cycle, broadly between about 57,000 and 29,000 years ago. Ireland was not yet in the fullest grip of the Last Glacial Maximum. Later ice would remodel and bury large parts of its surface, but current reconstructions indicate that, as the final British–Irish Ice Sheet expanded after c. 31,000 cal BP, ice caps and mountain glaciers grew in some Irish uplands while other ground remained exposed. Conditions varied greatly by region, altitude and century. It would be misleading to call either all Ireland ice-free or all Ireland ice-covered at c. 33,200 cal BP.
This was an age of pronounced climatic instability, long before the last Irish ice sheet reached its fullest extent. A widely used reconstruction places the major build-up of the British–Irish Ice Sheet after about 35,000–32,000 years ago, with maximum ice extent later, broadly between 27,000 and 21,000 years ago. That framework does not mean that every part of Ireland followed one timetable. Ice masses, margins, rivers, lakes and coastal environments altered at different times in different regions. Around the modern calendar label of 31,703 BCE, Ireland was entering a colder phase in which glaciation was becoming increasingly important, but its precise local reach remains debated.
The closest Irish evidence is dated in ranges rather than years. At Derryvree near Maguiresbridge, County Fermanagh, roadworks exposed organic freshwater silts and fine sands between two layers of till, the debris laid down by separate ice advances. A conventional radiocarbon measurement from the organic deposit was 30,500 ± 1,100 years BP. Such a result needs calibration, and at this age the calibration curve produces a broad interval rather than a precise answer. Published assessments place the Derryvree date broadly at about 36,900–31,700 calibrated years BP. Thus 31,770 BCE overlaps the possible age of that wetland, but the deposit cannot prove what happened at Derryvree, still less across Ireland, in that numbered year.
