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Cadair Idris
Cadair Idris wide view.jpg
Llyn Cau with the summit of the mountain to the right
Highest point
Elevation 893 m (2,930 ft)
Prominence 608 m (1,995 ft)
Parent peak Penygader
Listing Marilyn, Hewitt, Nuttall
English translation Chair of Idris
Language of name Welsh
Location Gwynedd, United Kingdom
Parent range Snowdonia
OS grid SH711130
Topo map OS Landranger 124, Explorer OL23
Easiest route Hike

Cadair Idris is a mountain in the historic county of Meirionnydd, Wales (or Gwynedd council area for local government purposes). It lies at the southern end of the Snowdonia National Park near the town of Dolgellau. The peak, which is one of the most popular in Wales for walkers and hikers, is composed largely of Ordovician igneous rocks, with classic glacial erosion features such as cwms, moraines, striated rocks, and roches moutonnées.


Cadair Idris means 'Idris's Chair'. Idris is usually taken to be the name of a giant or, alternatively, it may refer to Idris ap Gwyddno (or Gweiddno), a 7th-century prince of Meirionnydd who won a battle against the Irish on the mountain. Idris ap Gwyddno was in fact referred to as Idris Gawr ("Idris the Giant") in some mediaeval genealogies of Meirionydd.

The basic meaning of the word cadair (Middle Welsh/Early Modern Welsh kadeir or cadeir) is 'seat, chair' (borrowed from the Greek cathedra 'chair'). In place names cadair can mean 'stronghold, fort, fortress' or 'mountain or hill shaped like a chair'. The spelling cader represents a spoken variant of the standard form cadair.

It appears that Cadair/Cadeir Idris is the form used in the earliest Welsh-language sources. In a poem in his own hand in the second half of the 15th century, the poet Lewys Glyn Cothi wrote 'Dros gadair idris gedy' ('and then over Cadair Idris'). Around 1600, John Jones of Gellilyfdy referred to 'y mynydh neu bhan neu bhoel a elwir Cadeir Idris' ('the mountain, peak or hill known as Cadeir Idris').

The spoken form represented by cader had developed by the end of the Middle Ages and as a result the form Cader Idris was often used in English and Latin documents at that time and in more recent documents in English and Welsh.

In his Dictionarum Duplex published in 1632, John Davies of Mallwyd differentiated between the two words cadair ('chair') and cader ('fort, fortress'), referring to 'Cader Idris' and 'Cader Ddinmael'. That interpretation was followed by some later lexicographers, including Thomas Charles and Titus Lewis. It is now accepted, however, that cader is a spelling of a spoken form of cadair, and that there is no evidence that cadair and cader are separate words. But as it represents the pronunciation in the local dialect, the spelling Cader Idris is often seen in Welsh and English, and in June 2016 Snowdonia National Park decided to adopt that spelling on its signage, despite advice from the Welsh Language Commissioner and from Park officers, who favoured 'Cadair'. The local dialect form is also seen in the name of the local secondary school, Ysgol y Gader. The name of the mountain is typically spelt Cadair Idris on current maps. The summit of the mountain is known as Penygader ("top of the chair/stronghold").


The northern side of Cadair Idris viewed from Garth Gell (Bontddu)
Listed summits of Cadair Idris
Name Grid ref Height Status
Mynydd Moel SH727136 863 metres (2,831 ft) Hewitt
Cyfrwy SH703133 811 metres (2,661 ft) Hewitt
Craig Cwm Amarch SH710121 791 metres (2,595 ft) Hewitt
Gau Graig SH744141 683 metres (2,241 ft) Hewitt
Tyrrau Mawr SH677135 661 metres (2,169 ft) Hewitt
Craig-y-llyn SH677135 622 metres (2,041 ft) Hewitt

There are three main trails that lead to the top of Cadair Idris. The summit, which is covered in scree, is marked by a trig point. There is also a low-standing stone shelter with a roof.

Pony Path

The Pony Path (Welsh: Llwybr Pilin Pwn), which begins in the north from either Dolgellau or the Mawddach estuary, is the easiest but the longest of the main trails. Its length from the mountain's base is 6 kilometres (3.7 mi) with a 727-metre (2,385 ft) climb.

Fox's Path

The Fox's Path (Welsh: Llwybr Madyn),This is the most direct way to the summit as the trail leads straight up the northern face. The 3.8-kilometre (2.4 mi) ascent involves a climb up a 310-metre (1,020 ft) cliff-scree face. However, this part of the Fox's Path has been heavily eroded in recent years making the descent dangerous.

Minffordd Path

The Minffordd Path (Welsh: Llwybr Minffordd) starts on the southern side of the mountain near the glacial Tal-y-llyn Lake. Hikers using this ascent climb past Llyn Cau and along the rim of Craig Cau (rockwall) to Penygadair. Its length is 4.4 kilometres (2.7 mi) and involves two climbs of over 300 metres (980 ft).


Roche Moutonnée Cadair Idris
A roche moutonnée near Llyn Cau. The direction of the glacial movement was from left to right.

Cadair Idris is formed from a complexly fractured and folded pile of both sedimentary rocks and igneous rocks of Ordovician age. They comprise the mudstones and siltstones of the Ceiswyn Formation which form much of the southern part of the mountain, together with the varied rocks of the underlying Aran Volcanic Group. The steep cliffs rising above Llyn Cau to the north are formed from hard-wearing basalts and tuffs of the Pen y Gadair Volcanic Formation whilst those on the south side are acid tuffs of the Craig Cau Formation. The imposing cliffs which characterise the north side of the mountain are formed from a microgranite intruded into the Ty'r Gawen mudstones. In common with the rest of Snowdonia, the faulting and folding of this rock succession took place during the Caledonian Orogeny.

The crater-like shape of Cwm Cau has given rise to the occasional mistaken claim that Cadair Idris is an extinct volcano (though as noted above, some of its bedrock is volcanic in origin). This theory was discounted as early as 1872, when Charles Kingsley commented in his book Town Geology:

I have been told, for instance, that that wonderful little blue Glas Llyn, under the highest cliff of Snowdon, is the old crater of the mountain; and I have heard people insist that a similar lake, of almost equal grandeur, in the south side of Cader Idris, is a crater likewise. But the fact is not so.

The natural bowl-shaped depression to which the name of chair has been attached, was formed by a cirque glacier during multiple ice ages when snow and ice accumulated on these slopes partly through avalanching. The glacier was up to a square kilometre in size surrounded on three sides by steep cliffs the highest being the headwall. The chair of Cadair Idris is this bowl-shaped hollow in the bedrock formed by the movement of glacial ice across the floor of the hollow, abrading it and the lip over which it then flowed.

There are several tear-drop shaped knolls above the edge of Llyn Cau, each of which is a roche moutonnée formed by the abrasive action of the moving ice. The two highest lakes are Llyn Cau and Llyn y Gader; the latter is thought to have a maximum depth of about 13 m (43 ft).

Much of the area around Cadair Idris was designated a National Nature Reserve in 1957, and is home to Arctic–alpine plants such as purple saxifrage and dwarf willow.

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