Once known as Ynis Witrin (the Isle of Glass), Glastonbury in Arthur's time was an island in the midst of a huge marsh (the remains of a prehistoric inland sea). The island was composed of four hills, the most prominent of which was the Tor at the center. In Roman times (specifically 250-400 AD) the peat bogs surrounding the island were almost constantly flooded. The Romans began a program of drainage which ultimately led to the construction of sea walls in the tenth century. The Tor rises 518 feet above sea level, but seems a bit higher due to the fact that the lands surrounding it are below sea level.

"One of the Glastonbury traditions is that to renew and fulfill its spiritual purpose, it must once more become an island."
     Nicholas R. Mann, Glastonbury Tor: A Guide to the History and Legends

Often mentioned in Celtic legends are sacred hills that lead to the Underworld. Perhaps it is no accident that the Tor is believed to be riddled with caves thanks to the action of a spring beneath the soft lime-stone. The outlet for this particular water source is the White Spring (the water contains calcium carbonate, hence white). A companion spring, with an outlet not 50 yards away but from an altogether different source, is the Red Spring-- more commonly knonw as the Chalice Well (red due to chalybeate, or iron content).