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 South showing consists of slumped outcrops of white-weathering rhyolite ash and diatomaceous earth. The diatomaceous earth is exposed at the top of a 100-metre high bank that ends in Prentice Gulch. Microscopic examination shows that it contains abundant diatoms.