The Riverside pyrophyllite showing is in the gully of a small creek on the south side of the Thompson River, 14 kilometres northeast of Ashcroft. A quarry, 450 metres south of the Canadian Pacific Railway tracks at Semlin siding, joins two deep opencuts.

L. Stairnes was the original discoverer of the pyrophyllite deposit. The Juniper, Sonny, Shiela Fraction and Margaret claims were located over the deposit in 1947 and 1948 and surveyed in the autumn of 1951. A rough road has been bulldozed from the Semlin siding to the quarry (ca. 1951). Other work comprised a 27-metre trench south of the quarry and two smaller cuts, all on the Juniper claim. One small trench and a few pits are on the Sonny claim.

The hostrock is rhyolite porphyry of the Upper Triassic Nicola Group, which is schistose, greenish grey and contains quartz, orthoclase and minor albite phenocrysts in a fine-grained groundmass.

Pyrophyllite occurs with quartz, calcite and minor pyrite as wallrock alteration along a shear zone in the rhyolite porphyry. The pyrophyllite is yellow stained with small selenite crystals. In the quarry, the pyrophyllite is light grey-white but very iron stained.

A bulk sample was sent to the Bureau of Mines, Ottawa, in 1948 for grinding tests. The conclusions reached were that if impurities were removed prior to grinding, the product obtained was suitable for ceramics or as a filler (Minister of Mines Annual Report 1951, page A222). In 1950-52, shipments totalling 305 tonnes were sent to the Mountain Minerals Limited grinding plant in Lethbridge, Alberta.