The geology of Milford Sound
As you travel through the fiord on your Milford Sound Cruise you will view some of the most impressive valleys, cliffs and rock formations in New Zealand. The atmosphere in the Sound can feel almost eerie as the seemingly timeless, silent rocks stare down onto your Milford Sound sightseeing cruise.
The rocks of Milford Sound and the rest of Fiordland were formed over 600 million years ago and are known as the Western Province crystalline rocks, which are very hard and resistant to erosion.
The rocks of Fiordland were once part of an extensive “shield mountain range” located on the eastern side of the ancient continent of Gondwanaland. Over many 100 millions of years they have been eroded down, moved about by tectonic plate movements, remoulded and reformed to become what they are now down here in the South Pacific. While there is some granite in Fiordland, the most common rocks of Milford Sound are varieties of gneiss and diorite.
At Dale Point the Gneiss contains garnet crystals up to 2.5cm in diameter and at Poison Bay a little to the south the garnet crystals can be as large as 10cm in diameter. During past tectonic movements, rocks that were buried as deep as 20-40kms were subjected to immense heat and pressure causing minerals to re-crystallise.
Nephrite, jade, pounamu or greenstone was formed in this way about 200 million years ago and as mountains were uplifted the greenstone has become exposed. Most of the greenstone deposits at Milford exist in the Anita Ultramafics zone, which forms a stripe of very distinctive orange weathered rock that runs from Anita Bay to Poison Bay and is easily identified from the air.