Above the quarry workings, the mountain is doing something stranger and slower. In 1990 Moelwyn Mawr was designated a Site of Special Scientific Interest of national scientific importance, not for its slate but for its Pleistocene survivors. On the north-east flank, a terrain of patterned ground - vegetated stripes - has held its form since the last Ice Age. On the north side, a debris tongue formed by a rock glacier extends into Cwm Croesor: a slow-moving mass of frost-shattered rock and ice that crept downslope as the climate warmed, and stopped where you can still see it. These are textbook features of a periglacial landscape, frozen mid-gesture for the geological record.

High on the south-east face sits Llyn Stwlan, a glacial cirque lake enlarged by dam-builders in the 1950s. The reservoir is the upper half of Ffestiniog Power Station's pumped-storage scheme - Britain's first major one, commissioned in 1963. Water released from Stwlan plummets through hidden tunnels to four turbines at Tanygrisiau, generating 360 megawatts of power within sixty seconds of a switch being thrown. Each night, when grid demand sags, pumps return the same water uphill against a head of more than 300 metres. The mountain is, in effect, a battery: charged at night, discharged at peak. The lake itself looks like any other tarn until the visitor notices the water level has dropped twenty metres since morning.

The classic round on Moelwyn Mawr takes in its slightly smaller sibling, Moelwyn Bach, by way of the Craigysgafn ridge between them. A common starting point is Tanygrisiau, where walkers climb a disused slate incline before gaining the grassy western slopes of Mawr. The summit holds a trig point and a panorama that reaches from the Llyn Peninsula to the Rhinogydd in the south and the Glyderau across the valley to the north. From Mawr the ridge dips to a col and rises again to the rockier Bach. Descent runs back down the road from the Stwlan dam. Both the Snowdonia and Harlech Ordnance Survey sheets are needed - the route falls awkwardly across the join.

Croesor Quarry sits high above its valley, perched at a point where bringing slate down was nearly as difficult as cutting it. The Croesor Tramway, opened in 1864, dropped finished slate to Porthmadog by a series of inclines so steep that loaded wagons descended on cables, powering the empty wagons back up. Croesor closed in 1930. After the Second World War the quarry's underground chambers were leased to Cookes Explosives of Penrhyndeudraeth, who used them to store propellants - until the Central Electricity Generating Board realised in 1971 that a detonation under Moelwyn Mawr could damage the dams of its new pumped-storage scheme. Both reservoirs were drained. The explosives were removed at about 250 tons a week. Then the lakes refilled.

The trig point on Moelwyn Mawr looks down on a landscape laid out for reading. Tanygrisiau Reservoir glints below the eastern face. Llyn Stwlan sits in its bowl directly south. Beyond, the dark sprawl of the Rhinogydd cuts the southern horizon, and on a clear day the silver sea-light of Cardigan Bay edges into view past the Llyn. Look the other way and the Snowdon massif rises across the gulf of the Vale of Ffestiniog. This is a working mountain, scarred and instrumented, still earning its keep two centuries after the first quarrymen drove their iron wedges into the slate. The walker just borrows the view for a moment before the wind moves on.