The West Glamorgan Slant was opened in 1920 by the West Glamorgan Colliery Co to extract coal remaining on the south side of the abandoned Clyne Valley Colliery that had closed in 1915. It worked under leases from the Morris and the Beaufort estates. The company had been created in July 1919 by James Pridmore (1879-1953), the grandfather of the politician Michael Heseltine. The same company also opened and worked the nearby Rhydydefaid (or Ynys) slant from 1919 to 1921. In addition to coal, the Beaufort lease also granted the fireclay and ironstone.
The company struck the Yard and Yankee veins in May 1920 with the first coal shipped out in August 1920 via the Clyne Valley branch of the Mumbles Railway. Tonnages are known for the first three months of operation:
- August 1920 155 tons
- September 1920 136 tons
- October 1920 85 tons
The slant was abandoned on 21 December 1921 and the Beaufort lease was surrendered on 30 October 1924. An entry continued to appear in the Colliery Yearbook for 1923 and 1924 but this should not be taken to indicate that it was re-opened, rather (as can be demonstrated from entries for other companies) that the compilers often repeated information from year to year without updating it. However, the figure that is given for the number of men employed (12 underground and 14 on the surface) is likely to be a reasonably correct figure for 1920-21.
The slant was located in the area of disturbed ground below and to the north of the footpath leading through the woods from the Ynys Newydd car park to the cycle track near the fishing lake. A number of bell pits, probably from 18th-century coal working, can also be found in this area. Part of the course of the siding from the Clyne Valley branch railway to the Clyne Valley Colliery (opened 1903) can also be made out.