Perhaps the ultimate "holy ground" was the grove of the Druids, the site of ancient ritual. Their reverence for nature - and working within its boundaries - made them particularly respectful of the elements in states of betweenness. They loved the seashore, neither land nor sea; the fog or mist, neither water nor air; dawn and dusk, neither night nor day.
Their sacred branch was holly, neither bush nor tree. Their rituals revolved around the agricultural cycle of the year, and they led the people in the union of practical life and the earthly world with that of the spirit.