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.