 The formation is the southernmost of a string of preserved Eocene highland lakebeds in Washington state and British Columbia.The lake system, within the Okanagan highlands, extends from the Klondike Mountain Formation north approximately in to southern central British Columbia.Early dating of the formation was based primarily on identification and correlation of the fossils found in the Tom Thumb Tuff, with Joseph Umpleby in 1910 reporting a putative age of Early Miocene.