The Clayburn Company mines the Fireclay seam and Tertiary shale from a sedimentary sequence that lies on granitic rocks of the Jurassic to Tertiary Coast Plutonic Complex on Sumas Mountain. These shales are thought to be part of the Eocene Huntingdon Formation.

Three underground mines and at least two open pit mines produced shale for the plants to manufacture sewer pipe, flue-linings, face- bricks, refractories and special refractory shapes. Calcined shale and light-weight aggregate were also produced. No production figures are available.

The Fireclay seam was deposited in an arc-shaped basin that averages about 500 metres east and west. The seam consists of dark grey non-calcareous shale. A sample of the mine material showed good plasticity with an average water content of 14.5 per cent and shrinkage characteristic of 4.0 per cent. This material is classed as a moderately dense firing refractory clay.

Clayburn Industries Limited produces approximately 25,000 tonnes of fireclay annually from a series of pits on Sumas Mountain. Some is used to manufacture specialty refractory products and the remainder in production of face bricks. Clayburn also supplies shale for local cement plants.