Milford Sound Geology
The rocks in the Milford Sound, Fiordland area tell a remarkable story, the different types—sedimentary, igneous and metamorphic, and they all react in different ways to the forces of nature.
The Geological Story Starts: 400 Million Years to Make a Landscape
We’ll start by going back an incredible 400 million years, when the tops of these huge mountains were at the bottom of the sea, and in fact weren’t mountains at all, but as flat as a pancake.
To give you some idea of how long ago that was, in those days Africa lay over the south pole, the first tiny plants were learning to survive on land, and animal life was restricted to a few slimy blobs living in the sea. Over millions of years, more and more sediment and the remains of tiny plants and animals fell to the sea bed, until it was so thick that the lower layers were compressed into solid rock by the weight of those above.
Geology Transforming: Sediment Becomes Mountains
Here in the Eglington Valley, on the way into Milford Sound, it’s been estimated that this sedimentary rock was once 40,000 feet thick — that’s as high as a jumbo jet flies!
So how did a thick layer of sediment become a mountain range? Well, as the continents moved and shuffled their way around the globe, the rock lifted and split and splintered. In places it lifted right out of the water and was colonised by primitive land plants.
Around 200 million years ago there were quite a few volcanoes around too, which spurted liquid rock from the earth’s core out to the surface where it hardened into a dense crust of granite and basalt. Volcanic rock can be found all along this valley.
‘Oceanscape’ to Landscape: The Mountains Sink and Rise Again
By 50 million years ago, this whole region was mountainous and covered in primeval rainforest and swift rivers. But then, just as things might have started to look a bit familiar, it all gradually sank beneath the ocean again! More sand and mud was laid down on top of the submerged landscape, once again hardening into solid sandstone and limestone, and to this day you can still find ancient fossil seashells in the mountains.
Finally, about two million years ago, uplifts in the earth’s crust pushed Fiordland back above the waters again, with a new, tertiary layer of limestone and sandstone on top of the volcanic rocks and the older sediments.
Earthquakes and the elements shook everything around a bit and eroded much of the later sediments. Sandstone and limestone are softer than volcanic rock, so these sediments were swept from the hilltops and are mainly found in sheltered pockets and valleys. The volcanic rocks were exposed as rugged hills, and the odd fault line drove deep clefts and crinkles into the landscape.
‘Recent’ Geological History: Glaciers At Work
It’s only in the last million years that the glaciers came and did their bit. Fiordland was hit by repeated ice ages during that time, and as the ice pushed its way down from the frozen mountaintops it filled this valley to the brim, all the way back to Te Anau and Manapouri.
Other glaciers ran down towards the coast, eroding the valleys well below sea level, which eventually gave us all the fiords. In many cases, the water is actually deeper inside the mouths of the fiords than it is outside, because the ice piled up here, before melting when it was finally ejected into the water.
One other thing that the ice did was push big chunks of rock into different places. A glacier can lift boulders the size of a house and carry them down to other areas. About 10,000 years ago when the last round of glaciers retreated, they deposited great heaps of rock around their snouts, called moraines. You can see several of these smaller hills at Knobs Flat. The glaciers retreated back up the valley in stages, dumping a moraine at each stage all along the Flat there. A moraine looks more like a round hill than a big, craggy mountain, so keep an eye out for some on the way into Milford Sound.
Milford Sound Geology Today: Visible Evidence of a Rich History
The shaping of Fiordland is by no means over. Water, wind, rain and ice continue to reshape its contours. It stands to reason that there may be more glaciers to come, and maybe even one day this could all sink to the bottom of the sea once again! That’s not likely to happen for many millions of years, but make the most of it in the meantime!
All this activity has left us with a diverse range of rocks. If you’re out walking in Fiordland you’ll certainly come across volcanic granite, which is a hard, speckly rock that comes in various colours from grey to pink. Look out for basalt and andesite, which are hard, dark green rocks, and diorite with its shiny white speckles. When you pass through the Homer Tunnel you’ll see plenty of diorite giving the rock walls their magical glitter.
There’s a fair bit of white and grey marble along the coast, which was once mined, and you’ll see greywacke everywhere — that’s one of the sedimentary rocks that was formed on the old sea bed. Your average smooth, grey river pebble around here is greywacke. Then there’s mud stone, which is a soft, clay-like stone that unfortunately doesn’t build very strong mountains, and gives us the occasional landslip. But sometimes you also find coal seams beneath the mud stone.
Down in the valleys we also have the tertiary limestone, which tends to yield a lot of fossils, the remains of sea creatures that fell to the sea bed before it hardened. Limestone is also perfect for forming caves. The Te Anau caves were formed about 15,000 years ago, when the much older limestone was gradually washed away as the glaciers melted, to leave large caverns dripping in stalactites.
Fiordland Geology’s Most Famous Product: Greenstone
But the finest and most famous rock in these parts is greenstone. It’s one of the main reasons that Maori used to make the hazardous trek through these mountains. The translucent, jade-green rock they call pounamu is abundant around the Fiordland coast, which is one of only a few known pounamu sites in the country.
Pounamu is an important and highly prized commodity in Maori culture, and was used to make jewellery, weapons, tools and talismans. Many of the traditional designs have survived and nowadays, you can see beautiful greenstone carvings in most New Zealand craft and tourist shops.
There’s a theory that the local Maori discovered the treasure trove of greenstone while they were hunting giant Moa, long ago. They soon established a way through the mountains to collect the precious stone from places like Anita Bay at the head of Milford Sound.