The Ashnola Bentonite showing outcrops along the east bank of the Similkameen River at Ashnola Bend, 9 kilometres south-southwest of Princeton.

The deposit is situated near the eastern margin of the Princeton Basin, a northerly striking fault-bounded trough filled by Eocene volcanic rocks of mainly intermediate composition, comprising the Lower Volcanic Formation, and an overlying Eocene sedimentary sequence of sandstone, shale, waterlain rhyolite tephra (tuff) and coal, up to 2000 metres thick, comprising the Allenby Formation.

The showing is hosted in a shale member in the upper part of the Allenby Formation (Princeton Group), known informally as the Ashnola shale (Open File 1987-19). The deposit consists of a 20-metre thick section of siltstone, bentonitic siltstone and bentonite, exposed along the bank of the Similkameen River. The beds strike 035 degrees and dip 12 degrees southeast.

Exchangeable cation analyses and cation exchange capacity (CEC) in milli-equivalents per 100 grams on two samples are as follows (Open File 1987-19):

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Sample Magnesium Calcium Potassium Sodium CEC

C86-349A 6.5 16.0 1.0 24.3 35.4

C86-349B 8.0 10.5 1.1 28.8 48.2

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Both samples were taken from a 15-metre high bank of bentonitic siltstone with sodium-rich montmorillonite.