Klondike Mountain Formation

The Klondike Mountain Formation is an Early Eocene geological formation located in the northeast central area of Washington state. The formation, named for the type location designated in 1962, Klondike Mountain north of Republic, Washington, is composed of volcanic rocks in the upper unit and volcanics plus lacustrine sedimentation in which a lagerstätte with exceptionally well-preserved plant and insect fossils has been found, along with fossil epithermal hot springs.

The formation is the youngest in a group of formations which belong to the Challis Sequence rocks. The formation unconformably overlies rocks of the Eocene Sanpoil Volcanics and much older Triassic and Permian formations. The formation is bounded on its edges by a series of high-angle strike slip faults, which have contained the Klondike Mountain Formation in a series of graben structures, such as the Republic Graben. Public access to a fossilifrous outcrop at the north end of Republic is mediated by the nonprofit Stonerose Interpretive Center.

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