The Klondike Mountain Formation is an Early Eocene (Ypresian) geological formation located in the northeast central area of Washington state. The formation is comprised of volcanic rocks in the upper unit and volcanic plus lacustrine (lakebed) sedimentation in the lower unit. the formation is named for the type location designated in 1962, Klondike Mountain northeast of Republic, Washington. The formation is a lagerstätte with exceptionally well-preserved plant and insect fossils has been found, along with fossil epithermal hot springs. The Klondike Mountain formation is the youngest in a series of formations which belong to the Challis Sequence volcanics, and is the southernmost of the Eocene Okanagan Highlands paleolake lagerstätten. 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 fossiliferous outcrop at the north end of Republic is mediated by the nonprofit Stonerose Interpretive Center. Extent The formation is located in northern Ferry County, Washington, with the majority of the sedimentation in the Republic and Curlew Basins on the east and in the Toroda Creek area to the north west. The town of Republic, Washington is situated at the southern end of the formation, with outcrops within the city itself. The formation was named for Klondike Mountain which is just to the northeast of the city, and where the type locality is located. The Curlew basin is situated north of Republic, with the northern edge along the Kettle River and the community of Curlew, Washington near the northeastern edge. The formation is the southernmost of in the string of Eocene highland lakebeds that extend into British Columbia. The lake system, overlapping the modern Okanagan highlands, extends from the Klondike Mountain Formation northwest approximately in to central British Columbia. Closely correlated with and possibly representing the same lake sequence as the Klondike Mountain Formation are the Penticton Group's Kettle River, Marama and Marron Formations in the Boundary District along the Canada-United States border. Age Early dating of the formation was based primarily on identification and correlation of the fossils found in the Tom Thumb Tuff, with Joseph Umpleby (1910) reporting a putative age of Early Miocene. This date was based in examination of fossils by C. R.