theastrarium:

“Thresholds are border areas where one force, power or element encounters another. These meeting areas are potentially the most highly magically charged of all. Thresholds exist everywhere: the seashore, that transitional area where ocean meets land; the foot of mountains, where land begins to rise; and caves, the subterranean thresholds between Earth’s outer and inner powers.”

– Judika Illes
Encyclopedia of 5,000 Spells
http://amzn.to/1LbDYAz

Image Credit: Pierre Alain D