In the Quesnel region Tertiary volcanic and sedimentary rocks overlie Paleozoic and Mesozoic rocks of the Cache Creek and Nicola groups. These Tertiary rocks range in age from Paleocene to Pliocene but are dominated by plateau basalts of Miocene age and an underlying sedimentary succession including tuffs, conglomerate, sandstone and shale. Included in this sedimentary assemblage is a horizon of diatomaceous earth from which past production is recorded.

The Webster showing consists of crushed diatomite exposed over a distance of 34 metres along a road cut. The depth of the exposed diatomite is about one metre.