Pomona and Rona Islands are located in Lake Manapouri, on the South Island of New Zealand.
Both Pomona and Rona Islands are composed of light coloured, hard plutonic igneous rocks such as granite, granodiorite and diorite. These rocks were formed at the margin of Gondwanaland about 145 million years ago, at the end of the Jurassic Period. They cooled deep underground, beneath a volcanic arc. On the western edge of Pomona island you can see rocks which are more foliated ie they have a striped appearance, meaning they were formed during a period of tectonic unrest.
The dramatic landforms we see around Lake Manapouri were all formed in the last few glaciations. Both Pomona and Rona are made of hard rock. As ice moved over the top of them, it rounded the surface and produced the classic roche moutonneé shape. Ice also scoured and plucked the steep northern face of Pomona and the southern face of Rona.