Lake shales of the early Eocene Tranquille Formation, Kamloops Group preserve excellent plant and animal fossils especially insects. Resting at ca. 600 m a.s.l. on oxidized conglomerates, the lowest well-exposed 25 - 30 m comprise three stratigraphic zones and vary with respect to volcanic input, texture and alteration. The basal 12 m thick zone of shales interbedded with discrete volcanic ashes consists of five definable sedimentary units of which chocolate brown fissile shales in the middle of the zone contain the best preserved fossils. Mineralized root crowns occur in deformed yellow shales immediately under this unit. In the second zone, 12- 20 m above the base, brown crumbly to blocky fine sandstones are interbedded with thin shales including a basal massive sandstone in relatively sharp contact with shales below. Fossils are poorly preserved during interval of increased erosion and slope instability. Progressively deformed and altered beds occur ca. 20-60 m above the base and include chalky reddish and deformed shales interbedded with relatively massive brown sandstones. The shale and sandstone units are individually relatively thick (3.5 -7.5 m) presumably reflecting stable and less stable intervals. The silicified shale is rarely fissile, but is fossiliferous. Above 60 m limited exposures reveal that beds extend >100 m above basal contact and are fossiliferous but highly altered. Volcanic breccia disrupts and truncates the lake sediments at various thicknesses varying from about 30 m to 100+m and has thermally and physically altered the underlying lake sediments. Scattered outcrops reveal good preservation of plants and fish.

Damming of a paleovalley by a volcanic flow created of a relatively deep lake with apparent annual bedding. Considering wide variation of ash bedthickness and form over short distances, they were deposited as deep-water flows and from the water column. Coarse sediment deposition and thermal and physical deformation occurred in upper part of the sequence, nevertheless fossiliferous shale was deposited in quiescent intervals. In addition to two well-known localities representing the base of the sequence, tens of additional vertical metres are potentially available for sampling and study.