First, congratulations on the brave attempt at logging these long forgotten monuments on the bleak Berwyn mountains. In the 12th century this topography helped to defeat Henry 11 in his third failed expedition into North Wales opposed by Owain Gwynedd. Wet, exhausted, hungry, and harassed by Welsh guerrillas, Henry's vast international army went home never to return.
Second, on a point of Welsh orthography, 'Pistyll Rhaeadr' does not mean the 'Spout of Rhea - a Celtic land Goddess' which admittedly is a preferred romantic association.
Whereas 'pistyll' has come to mean 'waterfall' its origin is in the Celtic word and prefix, 'pis' which, not to put too fine a point on it, means 'piss' and refers to 'a stream of falling water' a notion I will return to shortly. 'Pistyll' originally meant therefore a 'tap or faucet' (pistol) and 'pistyllio': 'to gush out' the vernacular Old English 'pizzle'.
'Rhaedr' or 'rhaeadr' on the other hand has also come to mean 'waterfall' but its origin is in the Celtic 'rhuo' which refers to the noise of falling water so 'rhuo -dwr' ('dwr': 'water') > 'rhaeadr' which technically translates 'cataract' with 'rhyad' meaning: 'the sound of waves'. 'Rhuo' is related to 'rhuthr' and 'rhuthro': 'impetuosity, force, violence; a furious gushing forth'.
As a matter of interest only, 'rhuthr' is cognate with 'aruthr' and interesting metathesis of 'arthur' and the name of 'Arthur's' father 'Uthr' without the superfluous 'ar' prefix. While we are in the Arthurian realm, we can also look at 'Urien Rheged'. 'Rheged' is cognate with the variant 'rheawdr' and means: 'that which runs or flows' and is a metathesis of 'rhedeg': 'to run' whereas 'urien' is etymologically derived from 'urine' so 'Urien Rheged': 'a stream of piss' but this is undistinguished from 'the ejaculation of semen' and this is important because it represents a fertility metaphor. The fact is that the Arthuriad is 'awash' with hydronyms as fertility metaphors - but that's another story.
Hope you find this interesting and not annoyingly pedantic.
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