A type of Pleistocene-age (formed 1.8 million to 10,000 years before present) permafrost that contains a significant amount of organic material with ice content of 50– to 90 percent by volume. Thawing yedoma is a significant source of atmospheric methane.

A yedoma in Russia shows the thick layer of ice and carbon material exposed along this body of water. --Credit: Vladimir Romanovsky