At the Pinaus showing, waterlain, tuffaceous (rhyolite ash) shale and siltstone lie at the base of a rhyolite succession which passes upwards into rhyolite tephra and flows. The diatomaceous earth locally develops at the base of this succession.

These diatomaceous rocks are absorbant, light in colour and weight, and contain a mixture of tuffaceous debris and diatom filaments. The age of these rocks appears to be Miocene based on preliminary evaluations of well preserved fossil leaves and pollen grains (personal communication in 1996 with L. Donaldson, Okanagan College). This would make the rhyolitic succession part of the Miocene Chilcotin Group.