Iwakura is one form of worship within Koshintoism (Shintoism practiced prior to the introduction of Confucianism and Buddhism to Japan), which worships nature (worshipping of spirits, animism) that has been in Japan since the ancient times.
磐座（いわくら）とは、日本に古くからある自然崇拝（精霊崇拝・アニミズム）である古神道のなかの一つの信仰をさす。