Dryas

Dryas Part of the characteristic three-fold 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. The three-fold Dryas—Allerød—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; the Allerød Zone II; and the Younger Dryas Zone III. In north-western Europe Pollen Zone I is subdivided into a, b, and c. Zone 1b represents a proposed Bølling Interstadial, with Zones 1a and 1c referred to as Oldest and Older Dryas respectively.