Younger Dryas

The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief (1,300 ± 70 years) period of cold climatic conditions and drought which occurred between approximately 12,800 and 11,500 years BP (before present). The Younger Dryas stadial is thought to have been caused by the collapse of the North American ice sheets, although rival theories have been proposed.

It followed the Bølling-Allerød interstadial (warm period) at the end of the Pleistocene and preceded the preboreal of the early Holocene. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala. In Ireland, the period has been known as the Nahanagan Stadial, while in the United Kingdom it has been called the Loch Lomond Stadial and most recently Greenland Stadial 1 (GS1). The Younger Dryas (GS1) is also a Blytt-Sernander climate period detected from layers in north European bog peat.

The Dryas stadials were cold periods which interrupted the warming trend since the Last Glacial Maximum 20,000 years ago. The Older Dryas occurred approximately 1,000 years before the Younger Dryas and lasted about 300 years. The Oldest Dryas is dated between approximately 18,000 and 15,000 BP.