Slate quarrying was a major industry for many years in the Moelwynion. Moelwyn Mawr's flanks have several major quarries on them. To the west is Croesor Quarry perched high above Cwm Croesor. To the north west is Rhosydd Quarry on the col between Cwm Croesor and Cwm Orthin. Within Cwm Orthin, Conglog and Wrysgan quarries are located on the north slope of the mountain. Within Cwm Ystradau to the east lies Moelwyn Slate Quarry.