Dorothea Quarry, Nantlle - August 23, 1989
Foundations believed to be of an aerial cable way (blondin) engine house.
Dorothea Quarry located in the Nantlle Valley commenced working in the early 1820s, By 1848 it had become the principal quarry in the area, employing 200 men and producing 5000 tons of finished slate.
Slate production peaked in 1872 at 17442 tons. In the 1930s over 350 men were employed at Dorothea. Production fell after the start of World War II and the quarry closed in 1970.
Following closure in 1970 the site flooded and has become a popular, though unofficial, scuba diving location which has resulted in a number of fatalities over the years.
The quarry consists of six pits, the deepest dropping 106m from the surface. The slate veins here run vertically, allowing unusually deep vertical pits to be dug. Because the pits fall below the water table they needed to be constantly pumped to stay dry.
A Cornish Beam Engine buily by Holman of Camborne, Cornwall was installed in 1904, and is believed to be the most recently built Cornish Beam Engine to be remain in existance.
The engine was replaced by electric pumps in 1951, though it was reactivated for a while in 1956.
During the 1970s the engine was restored by a company which took its name Dorothea Restorations, however, changing ownership of the site meant that by the 1980s the Cornish Engine House and its engine has been left in limbo and is decaying despite being a grade I listed structure.