Five hundred million years ago the rocks beneath the Carneddau lay on a continental shelf bordering the Iapetus Ocean. The mudstones, sandstones and volcanic tuffs of the Ordovician built up in layers - the Nant Ffrancon Subgroup, the Llewelyn Volcanic Group, the Cwm Eigiau Formation - before the Caledonian orogeny shoved two continents together and lifted everything skyward. Glaciers did the rest. The last ice sheet retreated about 10,000 years ago, leaving behind smooth summit ridges, scree-rimmed eastern cliffs, and moraines damming small lakes in the cwms. Llyn Cowlyd, the deepest natural lake in Wales at 70 metres, sits in one of those glacial trenches. Aber Falls drops 37 metres off a hanging valley scoured clean by ice. The shape of the range is the shape ice leaves when it goes.

Above Bethesda, on the north-western slopes of Drosgl, there are clusters of stone hut circles and three cairns piled on the top of Moel Faban. They belong to an Iron Age settlement that endured for a thousand years - through Roman occupation and out the other side. The Romans built a road, Bwlch y Ddeufaen, across the northern flank of the range; you can still walk it. By the thirteenth century, English ambitions were closing in under Edward I, and the great castles encircling Snowdonia were going up at Caernarfon, Conwy and Beaumaris. The Carneddau remained Welsh in the way only mountains can: too high to garrison, too poor to bother taxing. Today the ridge above Aber Falls supports something even older than the hut circles - a herd of Carneddau ponies, semi-feral, hardy as goats, genetically distinct enough that researchers at Aberystwyth University in 2013 classified them as a unique breed. The heavy snows of spring 2013 killed about half the herd. The survivors are still up there.

The central peaks read like a roll of Welsh royalty: Pen yr Ole Wen, Carnedd Dafydd, Carnedd Llewelyn, Yr Elen, Foel Grach, Foel-fras. For a long time one summit was called simply Carnedd Uchaf, "the upper cairn." In September 2009, after a campaign led by the Gwenllian Society, it was renamed Carnedd Gwenllian. Gwenllian was the only child of Llywelyn ap Gruffudd, the last sovereign prince of Wales. Edward I had her, an infant of less than a year old, taken from Wales after her father's death and placed in a Gilbertine priory at Sempringham in Lincolnshire. She lived there for fifty-four years and died in 1337, never seeing the country she should have inherited. Seven centuries after her birth, a peak in her father's mountains took her name.

Llyn Ogwen, at the foot of the range, is one of the candidates for the lake in which Sir Bedivere is supposed to have thrown Excalibur after the death of King Arthur. The two small lakes nestled below the eastern cliffs of Carnedd Llywelyn were once believed to be haunted by deformed fish - heads with no bodies attached, allegedly lurking in their depths. A more solid local tradition records that the two great Meini Gwynedd boulders near the summit of Carnedd Llywelyn were carried there bodily in 1542 from one of those lakes; Henry VIII himself is said to have ordered the claim investigated and pronounced it true. Whether the king of England really cared what large stones did in Welsh mountains is another question. The boulders are there.

From a flight passing west toward Anglesey, the Carneddau show as a single great whaleback of green and grey, drained by valleys carved at right angles. The Conwy slips along the eastern edge; the Ogwen runs the southern flank; the Irish Sea opens to the north. The plant life on the high ground has to be ferocious to survive - prostrate willows hugging the summit ridges, Wilson's filmy fern hiding in damp crevices, Welsh poppies among the scree. Buzzards and peregrines work the cliffs. The whole 7,000-hectare estate has belonged to the National Trust since 1951, when it came to them in lieu of death duties on the Penrhyn family. Half of it remains common land. The shape is still the shape the ice left.