Diatomite occurrences in this part of the map sheet occur within strata of the Miocene Fraser Bend Formation of the Chilcotin Group, a dominantly volcanic assemblage which forms part of an overlap assemblage on older terranes.

The Prentice East showing is at the top of a steep bank, over a hundred metres high, composed of slumped outcrop of white-weathering rhyolite ash, bentonitic ash and diatomaceous earth, which occurs in the upper 20 metres. Microscopic examination shows that the material contains abundant diatoms.