As you travel through the fiord on your Milford Sound Sightseeing 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.
Milford Sound Glaciation
Glaciation in Milford Sound
There have been 12 major glacial phases during the last 2 million years. The last big freeze known as the Otiran Glaciation began about 80,000 years ago and kept the southern mountains ice bound until about 13,000 years ago. Ice descended the mountains and down the valleys forming rivers of ice up to 2000 metres thick.
On the east side of Fiordland the glacier tongues carved out huge trenches where the lakes Te Anau, Manapouri, Hauroko, Monowai, Poteriteri, and Hakapoua now reside. They show an amazing symmetry with the fiords on
the west side where the glaciers carved out the steep sided fiords and overlapped the coastline. Icebergs would have calved from floating ice cliffs where they meet the surging sea. When the glacial ice began to recede back up the fiords the glaciers left huge terminal moraine deposits, called "sills", at the entrances to the fiords.
At Milford Sound both the ocean and the fiord depth on either side of the sill is over 300m deep. The top of the sill it is only 27m deep. This effectively prevents ocean swells from entering Milford Sound. There is also evidence that the Milford glaciers have left five old valley floors on the bed of Milford Sound. Left behind also are the sheer cliffs, hanging
valleys and spectacular waterfalls for all to see on a memorable Milford Sound sightseeing cruise.