The mountains of Moelwyn Mawr and Moelwyn Bach lie in the Blaenau Ffestiniog area, famous for its slate-quarrying history. Our walk today visits both summits, passing through some of the quarry remains. We start from the village of Croesor and head gradually uphill along a track towards the ruins of the Croesor Quarry. Beyond the quarry, we turn to climb more steeply up the grassy north ridge of Moelwyn Mawr to reach its 770 metre top. We descend to the south, heading along the little ridge of Craig Ysfafn and then steeply down a rocky path to Bwlch Stwlan. From here a path rakes up the side of Moelwyn Bach to the 710 metre summit. An easy descent along the west ridge brings us to a quiet lane and hence back to Croesor.