The three cairns here are close to Llyn Llech Owen. 'Llech' is a slab of rock. So indulge me with this stone-related folklore of the lake.
In 1884 I took [the tradition] down from my grandfather, Mr. Rees Thomas (b. 1809, d 1892), of Cil Coll, Llandebie - a very intelligent man, with a good fund of old-world Welsh lore - who had lived all his life in the neighbouring parishes of Llandeilo Fawr and Llandebie.
The following is the version of the story (translated) as I had it from him: - There was once a man of the name of Owen living on Mynydd Mawr, and he had a well ('fynnon'). Over this well he kept a large flag ('fflagen neu lech fawr': 'fflagen' is the word in common use now in these parts for a large flat stone), which he was always careful to replace over its mouth after he had satisfied himself or his beast with water. It happened, however, that one day he went on horseback to the well to water his horse, and forgot to put the flag back in its place. He rode off leisurely in the direction of his home; but, after he had gone some distance, he casually looked back, and, to his great astonishment, saw that the well had burst out and was overflowing the whole place.
He suddenly bethought him that he should ride back and encompass the overflow of the wate as fast as he could; and it was the horse's track in galloping round the water that put a stop to its further overflowing. It is fully believed that, had he not galloped round the flood in the way he did, the well would have been sure to inundate the whole district and drown all. Hence the lake was called the Lake of Owen's Flag ('Llyn Llech Owen').
As Mr Rhys explains, this is a similar story to one explaining the formation of Lough Neagh in Ireland - it also has an overflowing well and a magic horse.