The 'Blue Lagoon' at Abereiddi/Abereiddy was formerly the main quarry pit of the St Brides Slate quarry, which was part of the Abereiddi Industrial Complex (NPRN 101219). The quarry is depicted as part of the 'St Brides Slate Quarries' is on 1889 Ordnance Survey mapping, but 1907 Ordnance Survey mapping depicts it as 'Porth-gain Slate Quarries'. The Porthgain Slate Quarries (NPRN 400062) extracted stone not only from the headland immediately above the village, but also from half a kilometre or so west at the headland of Penclegyr, and west again on this headland above Abereiddy village. Ruins survive overlooking the pool of the former site of the engine house and other buildings associated with the quarry operations. Part of Abereiddi industrial complex (NPRN 101219). The quarry was connected to Porth-gain harbour (NPRN 34343) by tramway (NPRN 400060). The Pit is now a tidal pool accesible to small boats and with a concrete slipway at the E side.

Sources include:
Ordnance Survey, 1889, 1st edition, 25 inch
Ordnance Survey, 1907, 2nd edition, 25 inch

T. Driver, RCAHMW, 23 Sept 2011.
J. Latham, RCAHMW 18 Nov 2014