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 Sandstonelaid 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 Sawddeinto 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. Large moraines occur to the east of the summit at the base of the scarp, and below the prominent peak of Picws Du as well as those damming the Lake.
Legend of Llyn y Fan Fach
A folklorelegend is connected with the lake, known as the Lady of the Lake. In the folk tale, a local young man, son of a widow from Blaen Sawdde agreed to marry a beautiful girl who arose from the lake, with the condition that he would not hit her three times. He complied easily because the girl was so beautiful, and they were happy for years putting up a house in Esgair Llaethdy near Myddfai, and bringing up a family there. The girl had very special cattle, traditionally still kept at Dinefwr, Llandeilo, and other animals. But over time the man did hit his wife three times, as gentle admonishments. Reasons for why he hit her vary, from the wife laughing at a funeral or crying at a wedding. Regardless, she had to go back to the lake according to the promise, taking the cattle with her. Among the cattle was a team of four oxen which were ploughing at the time. They followed too, plough and all, and, “they say, to this very day you may see a well-marked furrow running right across Myddfai mountain to the edge of Llyn-y-Fan-Fach, which proves this story is true.” But the mother returned sometimes to help and instruct her sons, and in particular one called Rhiwallon. In due course Rhiwallon and the other sons went to the court of Rhys Gryg of Deheubarth, where they became famous doctors who are known today as the Physicians of Myddfai. A number of their medical formulae remain in the Welsh manuscripts.
Access
The lake can be reached on foot via the water boardroad to the small dam on the lake. There is a car park at the bottom of the single track road reached by a poorly signposted road from Llanddeusant village. The track passes filter beds and associated buildings about halfway up the hill, and there is an unlocked windowless rescue shelter or bothy by the wall of the dam. The shelter is normally unlocked, and although there is a fireplace, there is no fuel visible nearby. There are no windows, so the stranded walker will have to rely on his own illumination. A path continues to the west up the slope to the escarpment, and wooden barriers have been erected here to move the path to one side and allow damaged turf to recuperate. The path follows the edge of the cliff, reaching the summits of Waun Lefrith and then Picws Du, both of which overlook the lake. There are many deep gullies leading down from the cliff top, and numerous rock outcrops near the top. The Beacons Way follows the ridge down from the summits above the lake, and forms an alternative route onto or off the mountain to the dam road. It arrives in Llanddeusant village more directly, and gives access to the Youth Hostel in the village.
Wildlife
There are numerous different species of bird in the area, and they include the red kite, common buzzard, kestrel, carrion crow, common raven and skylark to name a few of the most obvious residents. The former birds can often be found soaring on the updrafts near the cliffs as well as on thermals from the valley below. The red kite was previously restricted to this and adjoining areas in South Wales such as Mynydd Mallaen, mainly as a result of persecution by farmers and gamekeepers, but attitudes have now changed. It was thought that the bird preyed on game birds such as the red grouse, although the evidence showed that they survive mainly on carrion, like many other birds of prey. The kestrel and buzzard are widely distributed, but the raven is restricted to the higher mountains. There is a wide distribution of mammals such as field voles, foxes and badgers as well as many songbirds. The skylark is plentiful due to the extensive rough pasture present below the main peaks which allows ground nesting of the species. Pied wagtails are common near the streams and torrents running from the hill tops.
