Bentonite occurrences are common adjacent to the Fraser fault along the eastern margin of the Chilcotin Plateau. Bentonite occurs in Eocene volcanic rocks of dominantly rhyolitic composition which are correlative with the Kamloops Group east of the Fraser fault. The Fraser fault system itself was probably instrumental in the formation of the bentonite deposits in areas of Eocene volcanic rocks in that the movement of the fault system caused the development of basins in which the bentonite formed.

While the volcanic rocks of the region have been dated as Eocene in age by both radiometric and paleontological methods, the age of formation of bentonite by the hydrothermal alteration of tephra is not known. However, it is likely that bentonite formation occurred not long after volcanism had ceased.

The Grinder Creek bentonite showing occurs to the west of, and adjacent to, the Fraser fault. This fault has juxtaposed white bentonite-rich tuff layers with chert and argillite of the Cache Creek terrane. The bentonite layers are intercalated with maroon and brown basalt flows and breccia and stratigraphically underlie a well bedded sequence of Eocene sedimentary rocks.