The Abercorris quarry (also known as Cwmodyn) was a slate quarry worked between the 1860s and the early 1950s. It was located at Corris Uchaf about 5 miles north of Machynlleth, in Montgomeryshire Wales. The quarry was connected to the Corris Railway which carried its products to the Cambrian Railways at Machynlleth for distribution. The Corris Railway is a narrow gauge (2' 3") railway whose origins date back to the 1850's. The line was initially built as a horse-and-gravity-worked tramroad to carry slate from the quarries of Corris Uchaf and Aberllefenni in southern Merionethshire to the nearest navigable point on the Afon Dyfi (River Dovey), where it was loaded into ships and carried to its diverse destinations. In the 1860's the line ceased to deliver direct to shipping, and instead transhipped its slate to the standard gauge railway at Machynlleth, in western Montgomeryshire.
In 1878 the line was acquired by a London company, Imperial Tramways Ltd, and three steam locomotives and ten purpose-built passenger carriages were introduced, although due to a dispute with the quarry owners, passenger services were suspended until 1883. For the rest of the nineteenth century the Railway was very prosperous, developing a substantial tourist traffic.
The land occupied by the Abercorris quarry was leased to Thomas Green of London in 1863 who began quarrying operations, although the site was probably worked on a small scale before this date. In 1874 amidst a boom in demand for slate, the Cwmodyn Slate & Slab Quarry Company was formed, but demand slumped soon after and the company was sold at auction in 1878. Its new owner, J.W. Orchard began operations again by 1880 but this operation also failed and the quarry was closed by 1888.
Abercorris reopened in 1889, employing 40 men but again failed and was taken over in 1893 by W. John Lewis and Arthur T. Carr. This operation continued with limited success through to 1914 when it closed at the outbreak of the First World War. Following the war the quarry was reopened in 1920 under the ownership of T.O. Williams and C. Humphries. By 1928 this latest attempt had been wound up.
There was another small scale attempt to work Abercorris in the mid-1930s, and it was sporadically worked by the owners of Braichgoch quarry in the early 1950s, but this was the last time that slate was quarried as a commercial venture.