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.
It is a type of substratum worship of a very large boulder. Besides boulders, examples of nature worship vary greatly from the worship of mountains and forests, which are local Shinto deities, as forbidden land (the forest itself is the shrine, and the forest itself is the local Shinto deity), worship of kamunabi such as Mt. Miwa and Mt. Fuji, which is the worship of fire (volcanoes), to the worship of water falls and meteorological phenomena such as wind, rain and thunder.
Iwasaka is considered to be another form of worship associated with boulders. However, in contrast to iwakura, there are virtually no actual examples. Therefore, they may be considered one and the same. In Nihonshoki (Chronicles of Japan), it is distinguished from iwakura.
Therefore, it represents something different from iwakura, because 'saka' is the border marking the sacred realm, and the Chinese character 'magaki' used in himorogi (primitive shrine) also means border in the form of a fence, and the basis of forbidden land implies a border marking the 'sacred realm' or a border between 'the eternal realm and this world.'
In Shinto rituals the deity descended from its body, which is iwakura, onto yorishiro (object representing a divine spirit) (called himorogi), and yorishiro and kamui (deity) were used as the focus of the celebration. With time, as the main shrine, where the deity is considered to always reside, became permanently installed, the object of worship moved away from the body of the deity and changed to the shrine itself. However, in most cases the shrine is built at the original site of worship in Koshintoism from ancient times. Therefore, in many cases sacred trees and sacred spirit rocks decorated with shimenawa (sacred rice-straw ropes) are used as yorishiro, and they remain as they are within the grounds of the shrines.
Currently, the trend is that forests and trees, such as the sacred trees, or the broad-leaved evergreen trees, such as sakaki, used as yorishiro in rituals are called himorogi worship or himorogi, and worshiping of mountains, rocks, boulders as yorishiro are called iwakura.