Where is it written about the Red and White dragons?

Greetings :) Yesterday there was a very chilled atmosphere in Glastonbury as people followed the red and white dragon procession up the high street and onto the fair field where a ritual marking the beginning of Summer was acted out. Gwyn ap Nudd handed over to Gwythyr ap Greidawl. All well and good but I fear that most people there have no idea where the red and white dragons spring from. Certainly not Glastonbury. They come from the legends of Merlin ( or Myrddyn as this was his real name) and specifically the building of Vortigern's tower. Myrddyn commented on why Vortigern's tower could not be built due to there being dragons underneath. He also prophesised about the portents of these two dragons. Here 's a pic of Dinas Emrys in Snowdonia. It s a great walk and where the aforementioned encounter took place ;) The view from on top Blessings B.C.