Llyn Cau does superficially resemble a volcanic lake, and the rocks of Cader Idris (aka Cadair Idris) are volcanic, but they form part of the roots of a geologically ancient volcanic complex. Cader Idris itself is not itself a volcano, and its "crater" is a cwm (aka corrie or cirque), one of the classic landforms of glacial erosion. The caption may well have sprung from a historical catalogue description of Wilson's painting, "An English landscape, with the remains of a volcanic crater", but Tate Britain aren't alone in repeating this myth. My first reaction was that it showed what you get when you let art curators loose on geomorphological description. However, a little search of Newsbank and elsewhere found repeated examples of this claim.
"the dead volcano Cader Idris on the edge of Snowdonia"
- Arts: Beloved country: Romancing the stone, Jonathan Jones, The Guardian, July 5, 2001
"Cader Idris, once the biggest volcano in Europe"
- BBC Wales
"Cadair Idris ... an extinct volcano with Llyn Cau, its crater lake"
- Canolfan Corris blurb
"First up, Cader Idris, half the rim of a volcano extinct for 150,000 years: reached through shadowy deciduous woodland, it delivers views over glittery volcanic lakes"
- Escape: Europe: These routes were made for walking, Nick Redman, The Observer, August 29, 2004
150,000 years? The last volcanic activity here was in the Ordovician, some 450 million years ago! The myth undoubtedly stems from the general resemblance of Cwm Cau to a volcanic crater. This idea can be traced as far back as the description of Cader Idris Mntn in the 1860 Tallis's Topographical Dictionary of England and Wales. Tallis reports on the 1801 visit of a Mr Donovan 2, who correctly recognised the rocks as volcanic but wrongly concluded of Cwm Cau that
with respect to the crater itself, it appears very clearly to have derived its origin from the violence of an explosion upwards, in which a very considerable portion of the highest eminence was torn from its native bed of rocks, and thrown over the other parts of the mountain. In confirmation of this suggestion it should be stated that the summit of the mountain is covered with an immense wreck of stones, ejected, as it is presumed, from the crater at the time of the explosion. It would be difficult otherwise to account for the vast profusion of those stones scattered in all directions round the loftiest elevations, and which, from the confused manner in which they are dispersed, must have been thrown into their present situation by no small violence.
In contrast, Kingsley's Town Geology had it right.
I beg my readers to put out of their minds once and for all the fancy that in any known part of these islands craters are to be still seen, such as exist in Etna, or Vesuvius, or other volcanoes now at work in the open air. It is necessary to insist on this, because many people hearing that certain mountains are volcanic, conclude—and very naturally and harmlessly—that the circular lakes about their tops are true craters. 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.
By Kingsley's time, this had been established by Alfred Russell Wallace, whose Ice Marks in North Wales (With a Sketch of Glacial Theories and Controversies) (S124: 1867) was the first account of the many glacial features on Cader Idris such as striations and moraines.
However, despite the Victorian debunking, the myth looks likely to trundle on for some time to come. As Kevin Rushby wrote:
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.
- Rock legend: It may not be big, and most of the tales attached to it are tenuous at best, but Cadair Idris is all mountain to walkers, Kevin Rushby, The Guardian, October 1, 2005
1. It's relatively little known that Kingsley was a keen amateur naturalist - an interest reflected in the many biological threads of his The Water-Babies.
2. See Account of an extinct volcano in Britain, Retrospect of philosophical, mechanical, chemical, and agricultural discoveries, pub. J. Wyatt, 1809. "Mr Donovan" is Edward Donovan, writer, natural history illustrator, amateur zoologist and curator/owner of the London Museum and Institute of Natural History, which displayed his own extensive collection of specimens.