The lake is overlooked by several prominent mountain peaks, especially Picws Du and Waun Lefrith. Waun Lefrith is formed from the sandstones and mudstones of the Brownstones Formation of the Old Red Sandstone laid down during the Devonian period. Its southern slopes are formed from the hard-wearing sandstones of the overlying Plateau Beds Formation which are of upper/late Devonian age. It is those rocks which form vertical crags along the top edge of the scarp. The northern face of Waun Lefrith was home to a glacier during the ice ages which gouged out the cwm in which Llyn y Fan Fach now sits. This empties via the Afon Sawdde into the River Towy. The southern slopes drain via the Twrch Fechan, the Nant Menyn and Nant Lluestau into the Afon Twrch and so into the River Tawe.