It is composed of sedimentary rock which has been thrust up in an earthquake, resulting in a characteristic flat triangular ridge having the appearance of the kind of sail regularly used by Pacific explorers such as Kupe.

According to Māori mythology, the formation is the result of a sail-making competition between Kupe and his companion Ngake.

After the sails were made, they were hung out to dry and were said to have formed the rock formation, which was then named after the legend.

[1] On 7 March 2023, the name of the rock formation was officially gazetted as Ngā Rā-a-Kupe, following the enactment of the Ngāti Kahungunu ki Wairarapa Tāmaki nui-a-Rua Claims Settlement Act 2022.

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