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A cold autumn morning, with the Snowdonia mountains steaming with cloud. We were looking for a high and handsome walk, something tastily mountain-flavoured but without actually ascending too far. “Going in the Carneddau? Tops are all covered, rain’s on the way,” a hero of the hills predicted in the Betws-y-Coed sports shop. As so often in the mountains of Wales, however, he’d reckoned without the effects of local weather. We started under gloomy morning skies and finished in glorious afternoon sunshine. In between, there were the two secret lakes of Melynllyn and Dulyn.

You can’t see either from the upland car park at Llyn Eigiau, high above the Conwy Valley and bang in the middle of the Carneddau range. In fact, they lay well hidden until we had climbed the old quarry track round the shoulder of the tongue-tinglingly named Clogwynyreryr and were deep in the hidden valley behind.

Dulyn was the first to slide into view across the cleft, a dark sliver of water in a bowl of rock-scabbed cliffs 500ft high. But it was Melynllyn we came to first, skirting an old quarry building where a great cast-iron flywheel stood buried up to its axle in rubble. The slate around Melynllyn is studded with tiny particles of abrasive quartz; and first-class hones or whetstones were quarried here to sharpen the scythes and sickles of Victorian Britain.

The clear water of Melynllyn lay hidden until the last moment. As we gazed, a fish jumped and disappeared. A steep track led down to Dulyn, black and still under its cliffs. The twisted fingers of an aircraft propeller reached out of the water like a demon hand in a Tolkien setting. As many as 20 planes have crashed into the cliffs above Dulyn over the years, and their engines and wing parts still litter the rocks. It was a hauntingly beautiful place to sit before taking the long homeward path.

Start & finish Llyn Eigiau car park (OS ref SH 731662) Getting there Train (thetrainline.com; railcard.co.uk) to Dolgarrog Halt (4½ miles by footpath). Road: A5/A470 to Betws-y-Coed; B5106 to Tal-y-Bont; left at Talybont Farmhouse (just before bus shelter and Y Bedol/The Lamb PH); mountain road 3 miles to car park.

Walk (6 miles, moderate/difficult, OS Explorer OL17): Cross stile at east end of parking place (732663); follow paved path. Cross stile (727666); follow track past sheepfold and up shoulder of Clogwynyreryr for 1¾ of a mile to ruin near Melynllyn Reservoir (706656). Ignore footpath on map; continue along track to SE corner of reservoir (703658). Follow track skirting to right of crags, down to reach Dulyn Reservoir.

Follow path above bothy (707664), along hillside above Afon Dulyn. Pass Scots pine clump; cross first stream (709669), then fence by ladder stile. Cross Ffrwd Cerriguniawn (713671), and another ladder stile (715673). Cross Afon Garreg-wen (718675); then head right, aiming downhill for white dam a third of a mile away. Ford Afon Dulyn below dam (725675); follow track to Maeneira farm ruin (728673) and on to re-cross stile below sheepfold (727666) and return to car park.

Conditions a mountain walk — hill-walking clothes, boots, gear.

Online map, more walks christophersomerville.co.uk Lunch Picnic Accommodation Mairlys B&B, Betws-y-Coed (01690 710190; mairlys.co.uk; from £60 double), or Acorns B&B, Betws-y-Coed (01690 710395; betws-y-coed-breaks.co.uk; from £60) Snowdonia Walking Festival: October 16-18 2009 (snowdoniawalking festival.co.uk) More info Betws-y-Coed TIC (01690 710426; visitwales.co.uk); ramblers.org.uk