Oldest Dryas

The Oldest Dryas is a biostratigraphic subdivision layer corresponding to a relatively abrupt climatic cooling event, or stadial, which occurred during the last glacial retreat. The time period to which the layer corresponds is poorly defined and varies between regions, but it is generally dated as starting at 18.5–17 thousand years (ka) before present (BP) and ending 15–14 ka BP. As with the Younger and Older Dryas events, the stratigraphic layer is marked by abundance of the pollen and other remains of Dryas octopetala, an indicator species that colonizes arctic-alpine regions. The termination of the Oldest Dryas is marked by an abrupt oxygen isotope excursion, which has been observed at many sites in the Alps that correspond to this interval of time.

In the Alps, the Oldest Dryas corresponds to the Gschnitz stadial of the Würm glaciation. The term was originally defined specifically for terrestrial records in the region of Scandinavia, but has come to be used both for ice core stratigraphy in areas across the world, and to refer to the time period itself and its associated temporary reversal of the glacial retreat.

In the Iberian Peninsula, the glaciers of the Pyrenees, Sierra Nevada, Central Range, and Northwestern Mountains, which had almost entirely disappeared by 17,500 BP, began to advance once again. Between 16,800 and 16,500 BP, these glaciers abruptly advanced into montane valleys and deposited moraines near the moraines formed during the Last Glacial Maximum. These glaciers then began to oscillate between advance and retreat until a final glacial advance at 15,500 BP. A thousand years later, following a general glacial retreat, these alpine glaciers were relegated to cirques.



Flora
During the Oldest Dryas, Europe was treeless and similar to the Arctic tundra, but much drier and grassier than the modern tundra. It contained shrubs and herbaceous plants such as the following:
 * Poaceae, grasses
 * Artemisia
 * Betula nana, dwarf birch
 * Salix retusa, dwarf willow
 * Dryas octopetala

Fauna
Species were mainly Arctic but during the Glacial Maximum, the warmer weather species had withdrawn into refugia and began to repopulate Europe in the Oldest Dryas.

The brown bear, Ursus arctos, was among the first to arrive in the north. Genetic studies indicate North European brown bears came from a refugium in the Carpathians of Moldavia. Other refugia were in Italy, Spain and Greece.

The bears would not have returned north except in pursuit of food. The tundra must already have been well populated. It is likely that the species hunted by humans at Lake Neuchâtel in Switzerland by the end of the period were present during it. Here are other animals present:

Aves
 * Gavia arctica, black-throated diver
 * Podiceps nigricollis, black-necked grebe
 * Cygnus cygnus, whooper swan
 * Aquila chrysaetos, golden eagle