A series of shales, including a true fireclay deposit occurs in sedimentary rocks that cap the southwestern end of Sumas Mountain, just north of Kilgard. The Tertiary sedimentary sequence overlies granitic intrusives of the Jurassic to Tertiary Coast Plutonic Complex. This sedimentary sequence comprises more than 366 metres of interbedded shales, sandstones and conglomerates.
The surface of the basement intrusive rocks generally has a south to southwest slope with irregular, localized humps and hollows. One of these hollows forms the rather restricted basin that contains the fireclay. A zone of altered, highly kaolinized material between the Tertiary sediments and the underlying rocks suggests that a period of intense weathering of the basement material preceded the deposition of the sedimentary series. The thickness of this kaolinized zone varies, and is known to range up to 21 metres at the exposure at the portal of the Richmix Fireclay mine (092GSE005). The kaolinized material is exposed in the workings of the old Fireclay mine and the more recent workings of the Clayburn Fireclay mine (092GSE004). In all of the mines, the fireclay seam ends at the kaolinized zone, along the western limits of the workings.
The lower part of the Tertiary series consists of alternating beds of shale and sandstone with a few lignite seams. Higher up, the shale beds are fewer and thinner, and thick beds of conglomerate are abundant. The shales are thought to be part of the Eocene-Oligocene Huntingdon Formation.
The characteristics of the shale beds vary greatly and at least nine different beds have been worked. The fireclay seam is the most valuable of the shale beds in the formation. The seam occurs near the bottom of the series, occasionally lying directly on top of the basement series. The eastern limit of the fireclay seam occurs at the contact between the sedimentary series and the basement rocks. In this area, the fireclay outcrops at the surface with 10 to 21 metres of kaolinized altered material between it and the basement rock. The shale beds have been mined for use in brick manufacturing and as an additive in cement production.
The fireclay seam was deposited in a basin that averages about 500 metres east and west, and is of unknown length north and south. This basin is arc-shaped, concave to the west. The fireclay is classed as a moderately dense firing refractory fireclay.
Sumas Mountain is a 14-kilometre by 4-kilometre southwest to northwest trending height of land in the central Fraser Valley consisting of four peaks known as, from southwest to northeast, Eagle Mountain, McKee Peak, Taggart Peak and Sumas Peak.
McKee Peak is underlain by gently westerly dipping Tertiary sedimentary rocks of the Huntingdon Formation. A prominent bed of sandstone/conglomerate forms a cliff on the south-side of the peak that is clearly visible from Hwy No. 1. A sequence of clay, shale and sandstone located at the east end of the cliff, on the eastern and southeastern slopes of McKee Peak and west of the Sumas Mountain Road (MINFILE occurrences 092GSE004, 005 and 024) has been mined by underground and open pit methods for nearly a century. Material mined at the site include medium to dark grey clay and light grey to buff coloured shale and sandstone. Bricks and other cast refractory products have been made from the clay/shale since 1905 and more recently the shale has also been exploited for its alumina content in the manufacture of cement and the sandstone for its silica content in the manufacture of cement. Products have been manufactured in the past at Kilgard located on the south-side of Sumas Mountain, at the village of Clayburn on the north-side of the mountain and are currently manufactured at a plant in Abbotsford located approximately 8 kilometres west of the mine site. Four pits lie within the compact 600 by 1200 metre mine area centred on the main pit of MINFILE occurrence 092GSE024. From south to north the four pits are: Pit 1 (MINFILE 092GSE004) - a small inactive pit of medium to dark grey clay/shale. Pit 2 (MINFILE 092GSE005) - Clay/shale and sandstone are being mined in a pit approximately 75 x 150 metres in size. The medium to dark grey clay is being mined at the lowest elevation and lowest in the sequence. The portal used for past underground production is located at the southern limits of the pit. Mining in 2004 is developing the resource to the west of the portal and directly above it.
Pit 3 (MINFILE 092GSE024) - medium to dark grey clay and light grey to buff coloured shale and sandstone are mined from this main pit approximately 100 by 400 metres in size. Pit 4 (MINFILE 092GSE024) - the medium to dark grey clay is mined at this small 50 by 75 metre pit located approximately 500 metres northwest of the main pit (pit 3).
