Somewhere between Oswestry and Dolgellau the blue began. At least it did on that particular day: the grey of English summer replaced, quite suddenly, with a vault of deep magnificent blue; the sort of blue that fills you up with hope and confidence, ready to leap into your walking boots and - come on, dammit! - conquer Wales's 18th highest peak, Cadair Idris.
It doesn't look good on paper, I'll admit, that 18th business. Nor do the statistics - 2,928ft is barely a bunion on a Himalayan foothill. A quick pre-walk foray into the historical hinterland, however, had revealed a veritable Everest of legend, myth and folklore: Cadair Idris, it appears, has an enduring and peculiar fascination, so much so that this relatively minor summit commands more attention from walkers than any other Welsh mountain, bar Snowdon. I even came across descriptions labelling the peak "an extinct volcano, once the biggest in Europe". It sounded improbable, but it certainly got me hooked - Cadair Idris was a pocket Titan, a Celtic Krakatoa, and definitely worthy of investigation.
From the north side, sitting at the most comfortable vantage point - the patio at Olwen Evans's farmhouse B&B, Tyddyn Mawr - there's a view of a long rising pastureland that ends in a buckled ridge of mountain. The summit is 2,200ft over the valley - enough to generate some grandeur - and there are satisfying steep screes and a trio of high lakes.
The name itself means Arthur's Seat though this Idris may have been an ancient Welsh wizard rather than the Romano-British king. The wizardry connection might explain the legend that anyone who spends a night on the summit will either become a bard or go insane. With that in mind we set out early, taking what is known as the pony path. It's a gentle ascent initially, sharpening for the last 300ft on to the ridge. After a breather and a chance to work out the topography of the panorama - Snowdon almost due north, Barmouth down there by the sea - we took the path up to the cirque where there are wonderful views across the two tarns below.
One of the Victorian walkers who came up here looking for answers was Alfred Russel Wallace, a man whose theory of evolution had been rather eclipsed by Darwin's Origin Of Species (published a year after Wallace dreamed up the idea while in the East Indies). Wallace had always been interested in the Welsh landscape and in 1866 he began a series of walks determined to solve long-standing issues about how Cadair Idris had been formed.
From the summit looking south, it's easy to see why the volcano theory grew: the craggy lip of the mountain falls away in a broad curve, inside which is a larger and darker lake, Llyn Cau, looking like a very convincing example of a water-filled crater. Wallace, however, was a man who liked to settle arguments with facts (his bullish approach to data collection while in south-east Asia had included butterfly hunting with a shotgun). He took the Craig Cau ridge southwards, a classic walk along the edge of the precipice and down to what was assumed to be the lower lip of a crater. No it wasn't, Wallace deduced. Cadair was volcanic rock, but not a volcano: the craters were ice age scourings.
Wallace was right, but that hasn't stopped the story. It was just too good, too appropriate for this dramatic mountain. It seems to have been poor Wallace's fate to never quite be heard, whether on matters of natural selection or Welsh geology.
We chose not to take his route - one of the advantages of setting out from Olwen's farm (or the nearby Ty-nant car park) is that a circuit is possible. Up the pony path, over the summit, then down on what's known as the foxes' path. It's a very poor name: no fox would be foolish enough to tackle this 1,000ft of sliding scree hat drops you at the shore of Llyn y Gadair lake.
Once at the water's edge I proved Wallace's ice age theory correct by stripping off and plunging in for a very brief swim. There are, I can confirm, no traces of volcanic activity, no warm currents from the bowels of the earth; the lake is undoubtedly glacial. I'd planned to do a width, but in the absence of a cardiac massage unit on the far shore, I settled for a quick dip with a sharp scream.
Where to go
The walk is about a nine-mile circuit. A useful starting point is the Ty-nant car park, five miles south-west of Dolgellau. The OS map is Explorer OL23.