And he came towards a valley, through which ran a river; and the borders of the valley were wooded, and on each side of the river were level meadows. And on one side of the river he saw a flock of white sheep, and on the other a flock of black sheep. And whenever one of the white sheep bleated, one of the black sheep would cross over and become white; and when one of the black sheep bleated, one of the white sheep would cross over and become black. And he saw a tall tree by the side of the river, one half of which was in flames from the root to the top, and the other half was green and in full leaf. <PERSON> the Son of <PERSON>, The <PERSON>
The river crossing near the split tree is taken is the entry into the 'not-world'; the sheep cross too and fro and are changed, later the otherworldly <PERSON> won't cross the water himself but sends a page to collect his kill.
Elsewhere in the Mabinogion, water again is a transition of our world and the not-world, where <PERSON> is only vulnerable if caught between the worlds in a particular way:
The day after they came and looked at the bath. 351
“Wilt thou go into the bath, lord?” said she. “Willingly will I go in,” he answered. So into the bath he went, and he anointed himself. “Lord,” said she, “behold the animals which thou didst speak of as being called bucks.” “Well,” said he, “cause one of them to be caught and brought here.” And the buck was brought. Then <PERSON> rose out of the bath, and put on his trousers, and he placed one foot on the edge of the bath and the other on the buck’s back."
Thereupon <PERSON> rose up from the bill which is called Bryn Kyvergyr, and he rested on one knee, and flung the poisoned dart and struck him on the side, so that the shaft started out, but the head of the dart remained in.
The Mabinogion is based on tales found in books dating to 1350, but the tales themselves appear to date from around 1060 to 1200, and draw on pre-Christian tradition. 351