Myths of the Norsemen: From the Eddas and Sagas

It was, of course, essential that the tree Yggdrasil should be maintained in a perfectly healthy condition, and this duty was performed by the Norns, or Fates, who daily sprinkled it with the holy waters from the Urdar fountain. This water, as it trickled down to earth through branches and leaves, supplied the bees with honey.

From either edge of Nifl-heim, arching high above Midgard, rose the sacred bridge, Bifröst (Asabru, the rainbow), built of fire, water, and air, whose quivering and changing hues it retained, and over which the gods travelled to and fro to the earth or to the Urdar well, at the foot of the ash Yggdrasil, where they daily assembled in council.

Of all the gods Thor only, the god of thunder, never passed over the bridge, for fear lest his heavy tread or the heat of his lightnings would destroy it. The god Heimdall kept watch and ward there night and day. He was armed with a trenchant sword, and carried a trumpet called Giallar-horn, upon which he generally blew a soft note to announce the coming or going of the gods, but upon which a terrible blast would be sounded when Ragnarok should come, and the frost-giants and Surtr combined to destroy the world.