Part of the characteristic late-glacial sequence of climatic change and associated deposits following the last (Devensian) ice advance and prior to the onset of the markedly warmer conditions of the current (Flandrian) Interglacial. The type sequence was first described for Allerød in Denmark, and shows upper- and lower-clay deposits rich in remains of Dryas octopetala (mountain avens), and between them deposits of lake mud with remains of cool-temperate flora, e.g. tree birches. The colder Dryas phases mark times of cold, tundra-like conditions throughout what is now temperate Europe and climatic effects that were felt more widely (e.g. extreme aridity to the east of the Mediterranean). The three-fold Older Dryas–Allerød–Younger Dryas sequence forms Pollen Zones I, II, and III of the widely accepted late and post-glacial chronology of Europe. The basal, Older, Dryas deposit forms Zone I (approximately 15 500–14 700 years ago); the Allerød Zone II; and the Younger Dryas Zone III (12 900–11 600 years ago). The sequence of cool and warm episodes constitutes the Bølling-Allerød interstadial.