Geology
The crater-like shape of Llyn Cau has given rise to the occasional mistaken claim that Cadair Idris is an extinct volcano. 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 was formed by a cirque glacier during the last ice age when snow and ice accumulated in the corries due to avalanches on higher slopes. In these depressions, snow persisted through summer months, and becomes glacial ice. The cirque was up to a square kilometre in size surrounded on three sides by steep cliffs. The highest cliff was the headwall. The fourth side was the "lip" from which the glacier flowed away from the cirque. Over thousands of years ice flowed out through the bowl's opening carving the chair of Caldair Idris.
As the glacier eroded the lip down to the bedrock, there are several tear-drop shaped hills above the edge of Lyn Cau. These geologically important rocky outcrops are called roche moutonnée (English: sheep-like rock) and were formed from the abrasive action of the ice.
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 .
Read more about this topic: Cadair Idris