Stadial

If you walk along the beach in Cape Cod you will see gigantic boulders, detritus of the last ice age that nosed these huge rocks south and then abandoned them as the ice sheet retreated north. That Ice Age  was the most recent glacial period occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago. The maximum extension of the glaciers within this last glacial period was approximately 22,000 years ago. The main mass was called the Laurentide Ice Sheet; at some places it was up to 2 miles thick.

It radically altered the geography of North America north of the Ohio River. At the height of the Wisconsin Episode glaciations, ice covered most of Canada, the Upper Midwest, New England and parts of Montana and Washington. The Great Lakes are the result of glacial scouring and pooling; Niagara Falls is also a product of the glaciations, as is the course of the Ohio River, which largely supplanted the prior Teays River.

The Earth began to warm and vegetative and animal patterns changed as the glaciers receded. Then, rather suddenly, things changed again. Between 12,800 and 11,500 years ago it got suddenly cold again and abruptly reversed climatic conditions back to near-glacial state. A short such period of lower temperatures during a warming trend is called a "stadial."  This period is called The Younger Dryas stadial, "younger" because it was more recent than the one several thousand years earlier and "Dryas" after the arctic-alpine flowering plant Dryas octopetala, whose pollen is found in cores dating from those times. (During these cold spells, Dryas octopetala was much more widely distributed than it is today as large parts of the northern hemisphere that are now covered by forests were replaced in the cold periods by tundra.)

(The Dryas)

This new period had an impressive impact in North America. The original glacier period allowed the Bering strait to freeze and thus passage of people from Asia to the area of Alaska. The bridge regressed during the warming and may have made a comeback in the Younger Dryas. But the impact further south was devastating with the extinction of big mammals including mammoths, mastodons, giant ground sloths, American camels, horses and saber-toothed cats. More, it doomed those people who hunted them, the Clovis People.

The cause of the Younger Dryas is still under debate. A cataclysm is possible but not essential. The Little Ice Age--not a true Ice Age as it was mild--is conventionally defined as the period extending from the sixteenth to the nineteenth centuries, or alternatively, from about 1350 to about 1850--caused cultural disruption without much fanfare or notice. The relationship with CO2 is unknown.