The Klondike Mountain Formation in north-central Washington is an Early Eocene formation centered around Republic and extending into the Republic and Curlew basins. Its lower lakebed sediments and upper volcanic rocks are especially important for exceptionally well-preserved fossils, including plants, insects, fishes, and fossil hot spring deposits. Formed in ancient lakes within fault-bounded grabens, it is the southernmost of the Eocene Okanagan Highlands paleolake deposits. Dating places its fossil-bearing layers at about 49.4 million years old, with evidence for an oldest age near 51.2 million years. It also connects closely to the Republic district’s mining history through hydrothermal alteration and epithermal gold mineralization.