Cwm Ebol tramway
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Apparently once a building in the middle of the site, these walls are now buried by slate chippings to some depth. Was it a building with a deep cellar originally or was it engulfed as the quarry became worked out and waste piles grew ?
(click image to enlarge)
References
"The Slate Railways of Wales", Alun John Richards, pub. 2001 Gwasg Carreg Gwalch [Cymraeg] [English] ISBN 0-86381-689-4