According to Paul R. Davis, this is 'an underground stone quarry located in the north-facing escarpment of Craig-y-fron, north-west of Bala. The quarry was opened in 1865 to provide stone for building the nearby Methodist College (NPRN 96141) and consists of a single long and narrow chamber cut back into the sloping bedrock for approximately 30m. Pillars of unquarried rock were left to support the roof.'
Source: Paul R. Davis
M. Ryder, RCAHMW, 4th February 2019
