Llyn y Tri Greyenyn was a small lake not far from Cadair Idris in the old parish of Tal-y-llyn, Merionethshire (now Gwynedd) in north Wales.

[1] One of the earliest references to the lake dates from around 1700 when Edward Lhuyd sent a questionnaire to every parish in Wales to inquire about geographical features, place names, local traditions and the like (his ‘Parochialia’).

A version of the tale is recorded by Thomas Pennant in the second volume of his work A Tour of Wales (1781): On the left, is the rugged height of Cader Idris, pass near a small lake called, Llyn y tri Graienyn, or of the three grains; which are three vast rocks, the ruins of the neighbouring mountain, which sometime or other had fallen into the water.

[3]A tradition about how the lake was filled is mentioned by Lewis Morris in a poem that describes his journey from Ceredigion in 1750 to visit William Vaughan of Nannau.

He says that the lake – if the tale is true– was filled by the urine of a giantess or witch: Galw ym Minffordd yn frau i olchi trwm feddyliau yfodd y tafarnwr y gorau rhoes y gwaetha i ninnau oddiyno dan graig ddibyn trwy fwlch y tri graienyn a rhyfedd Iawn (os gwir hyn) lle pisodd gwiddon lonaid llyn[4] (‘We called readily at Minffordd / to wash our heavy thoughts.