VI. Milford Sound/Piopiotah: Geology and Glaciation

Sheerdown Peak (1,878m) and the sheer cliffs that rise over 1,700m from sea level. The Underwater Observatory is on the right and butress of Cascade Peak (1,209m) on left.
Sheerdown Peak (1,878m) and the sheer cliffs that rise over 1,700m from sea level. The Underwater Observatory is on the right and the buttress of Cascade Peak (1,209m) on left.

The Preserved Glaciation of the Ice Age

One of the very striking things about the Milford Road and Sound is the particular character of its glaciation.  I have looked at glaciation through the example of the Fox Glacier, 220km north-east of Milford Sound, in some detail (here).

 

The glaciation of the Fox Glacier is very impressive and very active but it nowhere reaches the sheer vertical audacity of Milford Sound.

A buttress on the rock wall of The Lion in Milford Sound.
A buttress on the rock wall of The Lion in Milford Sound.

The valleys of the Fox Glacier are U-shaped to an extent, but water and other forms of erosion soon chip away at that signature of recent glaciation. In Fiordland the U-shape of the valley is more pronounced and extreme and the depth of the valleys themselves outstrips anything the Fox Glacier has done.

 

Why is this? Clearly the rocks are different - Fox is uplifted and upturned greywacke and schists. Fiordland is primarily granites, diorites and gneiss.

Milford Sound rock wall exposed by a tree fall shows the clear makrs of the glaciers passing at least 20,000 years ago.
Milford Sound rock wall exposed by a tree fall shows the clear marks of the glaciers passing at least 20,000 years ago.

Fiordland is also not as high as the Alpine massif and consequently does not have the extensive permanent snowfields (névé) that are the source and sustenance of the great Southern Alps glaciers.

 

The Southern Alps snowfieds top out at Mts Aoraki/Cook (3,764m), Tasman (3,498m), Malte Brun (3,155m) and Elie de Beaumont (3,117m) and 20 other 3,000m-plus peaks. The highest mountain in Fiordland is Mt Tutoko at 2,723m. That is a thousand metres less than Aoraki/Mt Cook. In southern Fiordland the mountains are between 1,000 and 1,500 m high.

This section of side wall appears to steep even for moss. There is cncern that an overdue major earthquake could unleash a massive rockfall and tsunami in the confined waters of the Sound.
This section of side wall appears to steep even for moss. There is concern that an overdue major earthquake could unleash a massive rockfall and tsunami in the confined waters of the Sound.

In Fiordland there is consequently much less active glaciation than further north. Instead what remains are the valleys carved out of the harder rock by the super-glaciers of the successive Ice Ages.

 

As such Fiordland preserves spectacular examples of relict the geomorphology of the Last Glacial Maximum which was about 28-18,000 years ago in Fiordland (Dykstra 2012 PhD thesis above p. 71).

Looking WSW to the Footstool (635m centre) and Mitre Peak (R) (1,683m). Sheerdwon Pk is back L with Mt Gendarme ((1,931) and Lloyd Pk ((1,962m) in back centre R. (Possibly).
Looking WSW to The Footstool (635m centre) and Mitre Peak (R) (1,683m). Sheerdown Pk is back L with Mt Gendarme (1,931) and Lloyd Pk ((1,962m) in back centre R. Williamson Pt on extreme R.(Possibly).

Description of Milford Sound

 

These are the dimensions of Milford Sound: width at sea level 1.94km, total depth - summit to sea floor 2.17km, depth below sea level 291m, sill depth 71m, length 15.1km, fiord surface area 25sq km and catchment area 540 sq km,  (Dykstra 2012 p.77).

 

Milford Sound is a 17 km long glacial fiord with extremely steep bedrock walls extending up to two thousand meters above sea level, and dropping to nearly 300 metres below sea level.

 

The fiord is backed Mts. Pembroke (2,015 m) and Grave (2,225 m) to the north. Other peaks - Mitre (1,720m), Rover (1,524 m), the Elephant (1,508m), Mt. Philipps (1,446 m), the Lion (1,302 m), and Cascade Peak (1,209 m), form steep (>45◦) sea cliffs that plunge directly into the fiord.

 

The north face of Mitre Peak towers 1,680 m above the south shoreline of Milford Sound, contending for the title of highest sea cliff in the world, as it falls directly to the fiord in a horizontal distance of approximately 1 km.

 

See Dykstra, J.L., (2012) The Post-LGM Evolution of Milford Sound, Fiordland, New Zealand PhD Thesis, University of Canterbury  pp. 25-6.

It is hard to grasp the immense scale of Milford Sound (note the kayaks and our boat, the Milford Explorer). From L to R Sheerdown, Odyssey, Mt Ada and Mt Phillips.
It is hard to grasp the immense scale of Milford Sound (note the kayaks and our boat, the Milford Explorer). From L to R Sheerdown, Odyssey, Mt Ada and Mt Phillips.

The Volume of Rock Removed

The volume of rock removed to create Milford Sound is astoundingly large if difficult to calculate.

 

If we say the Sound is on average 1.5km wide at the level of its surrounding peaks  and 1.75km deep from peaks to the bottom of the Sound and is 17km long its head to the sill under the Tasman Sea this would give a volume of 44.6 cubic km.

 

That is 44.6 billion cubic metres of rock.

 

The specific gravity of granite is 2.7 which means a cubic metre of granite weighs 2,700kg or 2.7 metric tonnes. The weight of the volume of rock removed to create Milford Sound would be in the region of 120 billion tonnes.

 

And that is just for the main Sound itself and does not include the huge tributary glacial valleys that join it.

Milford Sound and its immediate mountains (NZ Topo Map).
Milford Sound and its immediate mountains (NZ Topo Map).

An open cast quarrying equivalent

One of largest open cast iron mines in the world is the Hull-Rust-Mohoning mine near Hibbing, Minnesota. In operation since 1895 the total ore and waste extracted is estimated at 1.02bn tonnes.

 

That is a pretty impressive rate of extraction at 1.02bn tonnes per 119 year period.

 

To excavate my approximation of the volume of Milford Sound at that rate would take 120/1.02=117.6 x 119 years = 14,000 years.

 

(This, of course, does not take into account differences of rock hardness between the iron ore bearing Mesabi Range in Minnesota where soft ore lay close to the surface and could be scooped from open pit mines and the Plutonic granites, diorites and gneisses of Milford Sound).

Looking  south-west down the Sound to Sheerdown Peak (1,878m) with Bowen Falls on left and terminus left centre.  The Darran Mountains visible through the notch  between Sheerdown and Barren Peak (1,5
Looking south-west down the Sound to Sheerdown Peak (1,878m) with Bowen Falls on left and terminus left centre. The Darran Mountains visible through the notch between Sheerdown and Barren Peak (1,561m left).

Blasting explosive quantities and costs

With harder rocks the rate of extraction slows while its expense rises as blasting operations become necessary. A paper on explosives used in Nigerian hard rock mines gives a ratio of about 0.34kg of explosives per ton of rock fragmented. That would be in the region of 40.8 billion kg (or 40 million tonnes) of explosive to remove Milford Sound.


(See Shehu, S. and Jethro, M., 2012 Preliminary Investigation into the Explosive Utilization of Selected Quarries in Kogi State, Research Journal in Engineering and Applied Sciences pp. 408-412.)

 

In 1990, 2.1 million tonnes of commercial explosives were consumed in the US, at an estimated expenditure of 3.5 to 4 billion 1993 dollars on blasting. Multiplied up by the 40 million tonnes of explosive required to carve out the Milford Sound gives a total explosives cost of US$71 billion.

 

Australia used about 2.5 million tons of explosives a year in its 2013 operations.

 

The amazing thing about Fiordland and the massive excavations achieved in its deep U-shaped valleys and sounds is that this was all done by ice, water and gravity.

The still waters of Milford Sound with the bulk of The Lion on the L and Mitre Peak on the R. Bowen Falls are just visible in the centre L. Photo taken near the inner sill of the firod where it plunge
The still waters of Milford Sound with the bulk of The Lion on the L and Mitre Peak on the R. Bowen Falls are just visible in the centre L. Photo taken near the inner sill of the firod where it plunges into the depths of Stirling Basin to nearly 300m.

The Glacial Periods

The glaciers of New Zealand are estimated to have begun their work around 2.5 million years ago with the Ross Glaciation. Since then there have been up to 20 glacial periods with nine in the last 700,000 years. The most recent glacial period was the Otira which took place between 75,000 and 14,000 years ago (see Te Ara: Glaciation).

 

The huge amounts of rock removed are in reality almost impossible to estimate because during this time the Alpine Fault was very active giving a rate of mountain uplift of 10mm a year in the Southern Alps - which over 2.5 million years adds up to an uplift of 25,000 metres (25km).

 

Rates of uplift in Fiordland are slower at 0.5-0.6mm a year but even this amounts to 1250m over 2.5m years.

Sinbad Gully (named by Donald Sutherland in his belief it contained diamonds) with ominous headwall cloud. The Gully is a spectacular U-shaped valley is home to a 12cm undescribed slug.
Sinbad Gully (named by Donald Sutherland in his belief it contained diamonds) with ominous headwall cloud. The Gully is a spectacular U-shaped valley and home to a 12cm slug.

The Shape of Fiordland's Glaciation

The pattern of glacial formation and erosion in Fiordland is thought to have come from the existing drainage patterns of Fiorldand.

 

Previous to glaciation Fiordland had a domed 'planation surface'.

 

Geologists deduce this  from the summit-level 'accordance' (the summits are more or less the same height) of the Fiordland peaks at between 1000 and 1,500m) and the radial nature of the glacial drainage from the upland peaks.

 

(See Augustinus, P.C., (1992), Outlet glacier trough size-drainage area relationships, Fiordland, New Zealand Geomorphology, (4), p. 348).

Mt Phillips (1446m) at the soouth west end of Milford Sound with Odyssey Peak and Mt Ada on the left. The foot of Mitre Ridge is on right leading up to The Footstool (not in photo).
Mt Phillips (1446m) at the south west end of Milford Sound with Odyssey Peak and Mt Ada on the left. The foot of Mitre Ridge is on right leading up to The Footstool (not in photo).

Pleistocene glaciers probably developed along pre-existing river valleys radiating from local topographic highs (Augustinus 1992). Few valleys were fault-controlled although the Te Anau/Manapouri/Lake Hauroko lake system follows the major Te Anau fault.

 

The first part of the Milford Road also follows the continuation of this major north-south fault system (The Hauroko/Te Anau/Darran faults) that defines the eastern edge of the uplifted Fiordland massif.

 

Glacial over-deepening is typical of most fiords and lakes in Fiordland but at Lake Hauroko it reaches its most extreme at 462 m below sea level.

Section of glacially modified rock wall near Stirling Falls showing glacial polish, stria (scratches and gouges), stichelwannen (curved grooves formed by water under immense pressure) and cavettos. Ic
Section of glacially modified rock wall near Stirling Falls showing glacial polish, stria (scratches and gouges), stichelwannen (curved grooves formed by water under immense pressure) and cavettos. Ice direction was from right to left.

A network of convergent ice rivers

Milford Sound was not just a single glacier but a network of glaciers with a possibly overlying ice sheet. The map below shows the major glacial valley systems that joined the Milford Sound.

 

From the north east these include the Stirling valley, the Harrison valley system, the Bowen valley, the Tutoko Valley, the Cleddau Valley system, the Arthur River valley and its seven tributaries from the north and the Joes River valley system from the south, and Sinbad Gully.

 

In total the current day catchment is 540 sq km and many of these valleys start in cirques at 1,500m in the west and 1,500m to 2,000m in the higher Darran Mountains to the east.

The inter-connected glacial valley system of Milford Sound (Google Earth with labels added).
The inter-connected glacial valley system of Milford Sound (Google Earth with labels added).
View down the Talbot River from unnamed peak pt 1655m. Lloyd Peak is on the far left; Castle Mount and Mt Elliot dominate the skyline at centre image

View down the Talbot River from unnamed peak pt 1655m. Lloyd Peak is on the far left; Castle Mount and Mt Elliot dominate the skyline at centre image. The Talbot River is a tributary of Joes River in aerial view above. Note the incredible steepness of valley slopes and striped patterning from tree avalanches. Also note the uniformity of peak heights. (From Danilo Hegg at Southern Alps Photography - click for link).


Mt Christina

Mt Christina from the south. Note the distinct series of terraced cuts into the shoulder of Mt Christina suggesting the successive passage of glaciers of different heights and sizes. (from Danilo Hegg at Southern Alps Photography - click for full screen version at his site)

Castle Mount

The waves of fractured and shattered aretes that lead to Castle Mount in Fiordland. (From Danilo Hegg at Southern Alps Photography - click for full screen version at his site)

Mt Wilmur, Mt Elliot and Mt Kepka

The beautifully formed glacial horns of Mt Wilmur and Mt Elliot in north western Fiordland. (From Danilo Hegg at Southern Alps Photography - click for full screen version at his site)

The coming together of the glaciers of the Bowen, Tutoko, Cleddau and Authur valleys at the head of the Milford Sound with the addition of the Sinbad and large Harrison valley glaciers a couple of kilometres further on would have created a massive convergence of ice that perhaps explains the overdeepening of the Stirling Basin to a depth below current sea level of nearly 300 metres.

 

Not that this network of glacial valleys was carved out in one fell swoop or scrape. Rather the process of gouging and plucking took place through successive glacial advances and retreats through the different glacial periods over two and a half million years.

 

The amazing thing is that when you look at Milford Sound and the surrounding mountains and valleys the glaciation seems so recent and so fresh.

The staggering view back to the boat terminus at Milford Sound and the peaks of the Darran Mountains with their immensely steep slopes carvbed out of hard Plutonic rocks. Bowen Falls to the left.
The staggering view back to the boat terminus at Milford Sound and the peaks of the Darran Mountains with their immensely steep slopes carvbed out of hard Plutonic rocks. Bowen Falls to the left and Cleddau River delta at R.

Successive glaciations

Each successive advance of valley glaciers progressively deepened and widened the valleys, lakes and fiords of Fiordland (Turnbull et al. 2010, Geology of the Fiordland Area, GNS p.5).

 

These produced the range of glacial features that are preserved in the hard rocks of the region - narrow aretes, armchair cirques and tarns, U-shaped valleys, fiords, glacial striations, roches moutonées, hanging valleys, striations, cavettos and the wonderfully named 'stichelwannen' -  curved grooves formed by water under immense pressure underneath and at the sides of a glacier.

The glacially-eroded horn-like summit of Odyssey Peak to the south west of Milford Sound.
The glacially-eroded horn-like summit of Odyssey Peak to the south west of Milford Sound.

Other features associated with the hard-rock glaciation of Fiordland are the existence of glacial horns, mammillation of mountain top relief (the rounding of peaks and ridge tops - from the Latin mammilla for nipple - and the existence of remnant cirque glaciers, particularly in the Darran Mountains.

 

Fiordland appears to have developed an extensive ice cap, as the glaciers have rounded off the mountain summits, as well as scouring deep valleys westward to well below today’s sea level (Te Ara Glaciation).

 

Who knows how thick the ice was at the different glacial maximums. The Tasman Glacier, the biggest in the Southern Alps, is 600m thick in places in our current interglacial period. Was Milford Sound completely full of glacier - which would imply an ice thickness of nearly 2km - or did successive glaciers ride lower in the already gouged out valley?

Rock spine on Milford Sound ridgetop showing evidence of ice-sheet erosion and mammillation of the Fiordland peaks.
Rock spine on Milford Sound ridgetop showing evidence of ice-sheet erosion and mammillation of the Fiordland peaks.

Backwards eating glaciers

Recent work on Fiordland glaciation has come to some surprising conclusions. Using a technique called thermochronometry a team of University of Berkeley geologists have concluded,

 

that ... the rock currently on the surface  [at Fiordland locations like Milford Sound] was about 1.5 miles (2 kilometers) underground when the glaciers began forming about 2.5 million years ago.

 

This work suggests that over the course of the successive glacial periods rock to the thickness of one and half miles/two kilometres has been removed. This can be seen to be the case in Milford Sound in as much as the Sound - from mountain top to undersea bottom is 2km deep.

 

Since then, the mountains rose as a result of tectonic activity, while the glaciers flowed downhill, scouring the landscape and gouging U-shaped valleys on their way to the sea.

Prof Kurt Cuffey Of UC of Berkeley overlooking the glacier-carved Bowen River drainage (middle), Mount Tutoko (far right) and Milford Sound (left) in Fiordland National Park of New Zealand. (Photo by
The glacier-carved Bowen River drainage (middle), Mount Tutoko (far right) and Milford Sound - Harrison Cove (left) in Fiordland National Park of New Zealand. (Photo by Johnny Sanders - click for link to UC Berkeley News)

They have also concluded that the fiord glaciers formed initially at their downstream mouths and that 'most of the valley-making occurred at the downstream mouths of glaciers' for 1m to 1.5m years'. After this the glaciers advanced up the valleys eating into their headwalls for another million years. 

 

Shuster et al (2011) suggest that 'major changes to the mountain topography [of Fiordland] essentially stopped about half a million years ago.'

 

(See D.Shuster ,K, Cuffey, J.Sanders, G. Balco (2011) Thermochronometry Reveals Headward Propagation of Erosion in an Alpine Landscape  Science 1st April 2011 p. 88.)

The view north up Milford Sound. Mitre Peak and Footstool on L.  The Lion (1,302m) and the Elephant (1,508m). on R.
The view north up Milford Sound. Mitre Peak and Footstool on left and The Lion (1,302m) and the Elephant (1,508m) on right.

Because of the hardness of the plutonic igneous rocks of Fiordland (and perhaps due to less fragmentary subduction faulting - the Alpine Fault lies to the west of Fiordland) subsequent erosion by wind, water, winter snow and freeze-thaw action has not substantially altered the landscape left behind by the last glaciation.

 

In particular, the lower loads of gravels and sediments have not filled in the deep fiords and lakes of Fiordland (which have an average depth of 440m/1,443ft).

 

In the Fox Glacier region side valleys are eroded out by torrential streams and creeks. In Milford Sound and along the Milford Road those same torrents and streams pitch out into open air as spectacular waterfalls that cascade over the lips of cirques and cwms left by long-disappeared glaciers.

Mt Pembroke (2,015m) and the Pembroke Glacier which disappears below 1,200m.
Mt Pembroke (2,015m) and the Pembroke Glacier which disappears below 1,200m.

Geology

Fiordland is a distinctive geographical region: it is the largest area of very strong crystalline rocks in New Zealand - plutonic rocks such as granite and diorite, and high grade metamorphic gneisses. These were uplifted as a single block and subsequently heavily glaciated. It is a huge, broadly domed mountain mass.

 

Milford Sound is part of the Authur River complex of Mesozoic plutonic rocks. The Milford Road from the head of Lake Te Anau goes through the Darran I-type volcanic suite.

Mills Peak (1,825m) and ridge that forms the spectacular spine above the Bowen Valley behind. Harrison Cove is in the middle foreground.
Mills Peak (1,825m) and ridge that forms the spectacular spine above the Bowen Valley behind. Harrison Cove is in the middle foreground.

These rocks were intruded over millions of years in different episodes between 500 and 150  million years ago (MYA) when this section of the Zealandia sub-continent formed the eastern edge of Gondwanaland (GNS, 2010, p.28.)

 

Intrusion took place as individual and related groups of plutons that make up the different suites of plutonic rock in the region. (Plutons 'a body of intrusive igneous rock ... that is crystallized from magma slowly cooling below the surface of the Earth' Wikipedia.)

Stirling Falls and the Little Matterhorn (1,508m).
Stirling Falls and the Little Matterhorn (1,508m).

The intrusions along the Milford Road took place in two distinct periods (see summary diagram below from NZ GNS 2010).

 

The Authur River complex is part of the second oldest intrusion in Fiordland and took place some 350 million years ago. The later Darran Suite was intruded 150 MYA.

 

The Authur River complex is formed of 'dioritic, gabbroic and minor granitic orthogneisses, metamorphosed in the Cretaceous to high-pressure amphibolite and granulite facies.'

The denuded ridge top to the south of Mills Peak showing ice sheet erosion picking out weaknesses in the Milford Sound orthogneisses.
The denuded ridge top to the south of Mills Peak showing ice sheet erosion picking out weaknesses in the Milford Sound orthogneisses.

'Orthogneiss' is gneiss - a metamorphic rock -  that is derived from igneous, as opposed to sedimentary, rock.

 

Gneisses are rocks that have been recrystalised due to immense pressure and temperature deep underground. Some gneisses are so old they make Fiordland's 350MYA tag seem 'young'. 

 

The Lewisian gneiss that forms most of the Outer Hebrides of Scotland contains rocks that are among the oldest in the world having been formed in the Precambrian period up to three billion years ago (see Wikipedia).

This summary diagram of the geology of Fiordland shows the development of the different complexes and suites over a 500 million year-plus period (from GNS 2010: The Geology of Fiordland).
Summary diagram of the geology of Fiordland showing the development of the different complexes and suites over a 500 million year-plus period (from GNS 2010: The Geology of Fiordland). The Darran Suite and Authur River Complex are the relevant ones.

At Milford the complex is formed from Milford Orthogneiss which contains 'banded gabbroic, dioritic and quartz dioritic orthogneisses, with subordinate ultramafic orthogneiss bodies' (GNS 42). These were intruded in the Paleozoic and Mesozoic eras.

 

The Darran suite was emplaced in the 140 million year period spanning the Late Triassic and Cretaceous during a renewed period of Gondwana margin subduction.

Vertically plunging fiord walls that continue for up to nearly 300m in the deepest parts of Milford Sound. A succession of Ice Ages and their glaciers ground out these huge valleys over millions of ye
Vertically plunging fiord walls that continue for up to nearly 300m in the deepest parts of Milford Sound. A succession of Ice Ages and their glaciers ground out these huge valleys over millions of years working backwards from the sea.

The suite comprises ... gabbros and diorites with subordinate granitoids and minor ultramafic rocks, typical of Phanerozoic convergent margin magmatic arcs' (GNS, 2010 p.34).

 

('Ultramafic' in effect means rocks from the Earth's mantle - that part of the Earth between the crust and the inner core.)

The Stirling Falls dwarfed by The Elephant L (1,508m) and the 1,500m ramparts of The Lion. A fierce wind was blwoing down the Sound from the Tasman Sea creating this dramatic light.
The 155m Stirling Falls are here dwarfed by The Elephant (1,508m) (left) and the 1,500m ramparts of The Lion (right). A fierce wind was blowing down the Sound from the Tasman Sea creating this dramatic light.

From an engineering perspective the geology of Fiordland is critical - for road builders and dam constructors and tunnel borers for example. It can be summed up thus:

 

Fiordland [rock-types] are in general strong to very strong ... capable of supporting very steep slopes. Quaternary glacial erosion and present-day erosion processes ... of steep slopes and high rainfall have removed most surficial weathered material, so [exposed] plutonic rocks tend to be fresh and hard. (GNS, 2010).

The staggering vertical walls of The Lion that rise 600m (2,000ft) vertically from water level. See the 155m Stirling Falls to left for scale.
The staggering vertical walls of The Lion that rise 600m (2,000ft) vertically from water level. See the 155m Stirling Falls to left for scale.

Spot the Difference: 1920s and 2010s

The sheer hardness of the Fiordland rocks was brought home to me by a strange coincidence. As we drove down the Milford Road in March 2014 I took as many photos as I could. One of these was of the Homer Hut near the eastern portal of the Homer Tunnel.

 

Months later as I was working on these web pages I came across a black-and-white photograph from the 1920s that was was almost exactly the same (see the Alexander Turnbull Library).

 

I was surprised just how little the scene has changed in nearly a hundred years.

 

The details of the rocks on the ridge lines are remarkably similar attesting to the  slow pace of erosion in this area with over 6 metres of rainfall a year, much of which falls as snow above 1000m.

 

I was also interested to compare the colonisation of vegetation on the scree slopes in both photographs. This looks like it has changed little and again attests to the slow pace at this altitude at which the progression of plant species on the bare, hard-rock screes develops (see photos below).

Spot the Difference: 2010s and 1920s photos

By complete chance the photo above and this one are almost the same but separated by nearly 100 years. View looking up the Homer Valley, showing the Homer Hut in the foreground, photographed in the 1920s by Algernon Gifford (Turnbull Collection - click)
By complete chance the photo above and this one are almost the same but separated by nearly 100 years. View looking up the Homer Valley, showing the Homer Hut in the foreground, photographed in the 1920s by Algernon Gifford (Turnbull Collection - click)
McPhersons Falls and Homer Hut  from the bed of the Hollyford River with soouth-eastern spur of Mt McPherson (1,931m) and the Homer Saddle on the left from the Milford Road.
McPhersons Falls and Homer Hut from the bed of the Hollyford River with south-eastern spur of Mt McPherson (1,931m) and the Homer Saddle on the left from the Milford Road.

To return to the main geological narrative, landslips are common on steep and over-steep slopes in Fiordland and facilitated by earthquake shaking. Rock weakness is increased where foliation (repetitive layering caused by differential shear pressure) and faulting are present.

 

Faulting was a particular issue in the Homer Tunnel because the faults allowed the ingress of thousands of litres per hour of freezing snowmelt water from above (see my Milford Road page).

The vast bulk of The Lion (1,502m) - the Stirling Falls to right are 155m for scale.
The vast bulk of The Lion (1,502m) - the Stirling Falls to right are 155m for scale.

Tectonic activity in Fiordland

Tectonic activity has taken place in Fiordland at varying intensities for over 400 million years as part of both the margins of the Gondwanaland supercontinent and much later as the Zealandia mini-continent was carried eastwards on the edges of the Pacific and Australian plates.

 

The Alpine Fault is the major area of tectonic activity in the modern era. Here the major forces are very obligue with dextral (right-lateral) strike-slip movement at rates up to 31 mm/year.

 

That means that the Australian plate is slipping north-eastwards against the Pacific plate's south-westward slippage. At the same time the Australian plate is being subducted (forced under, submerged) underneath the Pacific plate.

The 155m Stirling Falls as it tumbles into Milford Sound from the hanging valley above.
The 155m Stirling Falls as it tumbles into Milford Sound from the hanging valley above.

In contrast to the Southern Alps where there is 10mm a year uplift and 10mm year denudation, the Fiordland Mountains are characterized by relatively low rates of uplift at 0.5-6mm a year (less than a tenth of the rate of the Southern Alps) and low rates of erosion.

 

Sediment yields of the Cleddau River near Milford Sound are approximately 600 tonnes per square kilometre per year (t/km2/y) compared to yields of over 30,000 t/km2/y in the areas of active glaciation further north on the West Coast.

 

(See Dykstra, J.L., (2012) The Post-LGM Evolution of Milford Sound, Fiordland, New Zealand,  PhD Thesis, University of Canterbury p.13-17.)

 

The spectacular hanging valley above the Stirling Falls at Milford
The spectacular hanging valley above the Stirling Falls at Milford Sound.

This in effect means that New Zealand's fiords are not filling up quickly with sediment:

 

'Basin sedimentation rates for all the mature fiords of Fiordland (including Milford Sound) are extremely low by global standards, even compared to the fiords of Norway ... despite similar resilient bedrock geology, and little contemporary glacier cover,' (Dikstra, p.78).

Mt Pembroke in evening light with the lush temperate rain forest growth of Harrison Cove in the foreground.
Mt Pembroke in evening light with the lush temperate rain forest growth of Harrison Cove in the foreground.

 This slow to very slow rate of infilling is largely accounted for by the fiords, 'relatively small catchments, lush forest growth, sediment traps in upstream lakes [Lake Ada on the Authur River, for example] and erosion-resistant bedrock' (Dykstra p.90).

 

However while water-borne sediment yields are low earthquakes develop different 'sedimentation processes', namely rock and landslips.

 

The influence of a tectonically active plate boundary on fiord sedimentation is clearly evident in the Fiordland landslide record, particularly at Milford Sound, where extreme topography combines with seismic activity to cause mass wasting; at least 30 very large (...) post-glacial landslides are preserved in the Milford catchment (Dykstra p.94).

The Bowen Falls empties the spectacualr handing valley of the Bowen River. Note tree fall scar to left and the paucity of rock sediment and debris at the bottom of the falls.
The Bowen Falls empties the spectacular handing valley of the Bowen River. Note tree fall scar to left and the paucity of rock sediment and debris at the bottom of the falls.

The presence offshore of the Alpine Fault creates large amounts of earthquake activity. Major Fiordland earthquakes occurred in 1938 at Charles Sound (Magnitude 7), in 1988 at Te Anau  (M6.7), in 1976 off Milford Sound (M6.5), and in 2009 at Dusky Sound (M7.8). There was a 'strong'  magnitude 4.3 earthquake off Milford Sound the month before we went there.

 

A pictorial image of the process of subduction and seismic activity is presented below from GNS 2010.

Diagrammatic representation of modern tectonic and seismic activity in Fiordland. From GNS 2010 with place name additions.
Diagrammatic representation of modern tectonic and seismic activity in Fiordland (from GNS, 2010 with place name additions).

Looking at Milford Sound cast in deep shadow and wreathed in cloud the rock formations and topography look implacably resistant to change. Its brutal geography feels like an end point, a full stop after a titanic battle.

 

But the world around is in a state of processual flux. Fiords are but a stage of a process that leads inexorably to another stage.

 

The dry valley ground out by successive glaciers and over-riding ice-sheets over millions of years becomes a lake or saltwater inlet in the inter-glacial periods as the ice melts and the sea level rises. In the inter-glacial periods and after the last glacial maximum (and who's to say another Ice Age is not on the way) the lake or fiord begins to fill up with sediment.

Conceptual fiord evolution model for New Zealand (from Dykstra 2012 p. 89) showing evolution of fiord forms from tidewater glaciers to infilled coastal plains over a 20,000 year period.
Conceptual fiord evolution model for New Zealand (from Dykstra 2012 p. 89) showing evolution of fiord forms from tidewater glaciers to infilled coastal plains over a 20,000 year period (Click for link).

The retreating glacier itself leaves terminal and lateral moraines at various stages, depending on the speed of, and halts in, the retreat. Winter freeze-thaw and all-year round round water erosion do their bit. And in areas of seismic activity great landslides are loosed by earthquakes.

 

New Zealand's biggest documented landslide is the Green Lake slide that took place 13,000 years ago. This shifted 27 cubic kilometres of hard rock and covered an area of over 45 square kilometres (Te Ara).

 

Eventually fiords get filled in to become flat valley bottoms and coastal plains (see Dykstra's 2012 conceptual New Zealand evolution fiord model above). It's just a matter of time, of the development of the next stage in the constant flux and change on and below the Earth's surface.

Vulcan 3-D model showing submarine avalanche sediments in the Stirling Basin of Milford Sound (Contour interval 100m) using subaerial and bathymetric Digital Elevation Models (from Dykstra 2012 p.104)
Vulcan 3-D model showing submarine avalanche sediments in the Stirling Basin of Milford Sound (Contour interval 100m) using subaerial and bathymetric Digital Elevation Models (from Dykstra 2012 p.104 - click for link)).

Dykstra's work for me is fabulous because it provides such clear images of the profile of Milford Sound underwater - something I had searched for without success on the Internet.

 

It shows the huge Stirling Basin which stretches from the head of the Sound northwards for 8km reaching a depth of nearly 300m. The bottom at the southern end slopes upwards to the Cleddau River delta with thick layers of landslide debris.

The plunging vertical wall of The Lion that goes straight down nearly 300m underwater.
The plunging vertical wall of The Lion that goes straight down nearly 300m underwater.

On either side of the Stirling Basin the mountains and their steep to over-steep to vertical walls tower over the fiord. The rock walls plunge down nearly 300m in places (see the vertical wall on the buttress of Mitre Peak and the wall of The Lion opposite in the bathymetric digital elevation model above).

The Stirling Basin comes to an abrupt halt at the inner sill which is made up of landslide debris and glacial till. Dykstra speculates that the retreating glacier came to a temporary halt at this point. Beyond this seaward the fiord is around 130m deep.

Survey of sediments in the southern section of Milford Sound showing profile of fiord (Dykstra 2012 p. 167 - click for link).
Survey of sediments in Milford Sound showing profile of fiord (Dykstra 2012 p. 167 - click for link).

The Shape of the Milford Glacier

Dykstra's (2012) work on Milford Sound shows the extent - 4km approx - to which the glacier that formed the Sound reached out beyond the current entrance to the Sound (see diagram below) to the edge of the narrow continental shelf on the Fiordland coast.

 

This was possible because the sea level was 110m or more lower and the glacier's tongue was advancing over ground above sea level. The glacier left a massive terminal moraine complex that is now 55m under the Tasman Sea and lateral moraines that make up the seaward end of Yates Point. These have been aged at approx 18,000 YA (see p.170).

St Anne's Point outside Milford Sound. When Ice Age sea levels were 120m lower the Milford Glacier exteneded to the edge of the continental shelf, 4km from the fiord mouth.Rising inter-glacial sea lev
St Anne's Point outside Milford Sound. When Ice Age sea levels were 120m lower the Milford Glacier extended to the edge of the continental shelf, 4km from the fiord mouth.Rising inter-glacial sea levels forced it backwards.

As the glacier retreated from its last glacial maximum 4km west of the Sound's mouth a pro-glacial freshwater lake was probably formed in the Entrance Basin.

 

As the glacier retreated further and the massive pressure of ice was removed from the towering walls of the Sound these became 'more susceptible to failure, with landslides contributing to thick units of diamicton [poorly sorted muddy/sandy sediments] which were left to blanket the underlying bedrock' (Dykstra p.174) .

Hypothetical profile of the Milford Sound glacier at the Last Glacial Maximum showing se level at that time and a profile of the developing fiord bottom. From Dykstra p. 180
Hypothetical profile of the Milford Sound glacier at the Last Glacial Maximum showing sea level at that time and a profile of the developing fiord bottom (From Dykstra 2012 p. 180).
Yates Point to the north of the Sound entrance is made of lateral moraine deposits laid down by the Milford glacier as it moved out to the edge of the continental shelf when sea levels were 110m lower
Yates Point to the north of the Sound entrance is made of lateral moraine deposits laid down by the Milford glacier as it moved out to the edge of the continental shelf when sea levels were 110m lower at the Last Glacial Maximum.

The glacier retreated further and left the sill that is apparent midway down the Sound between the Dale and the Stirling Basins. This would have been caused by a temporary halt in the glacier's retreat allowing the accumulation of a mid-point terminal moraine.

 

The early part of the retreat to Dale Basin probably occurred at a minimum rate of 2.4km per thousand years. Thereafter the rising sea level in the deep Stirling Basin would have created buoyancy-driven iceberg calving and a rate of retreat in the region of 8km in 500 years.

Diagram and photograph showing the extension of the Milford Sound glacier beyound the Sound's mouth across the Alpine Fault and into the present day Tasman Sea. (From Dykstra 2012 p.169).
Diagram and photograph showing the extension of the Milford Sound glacier beyound the Sound's mouth across the Alpine Fault and into the present day Tasman Sea. (From Dykstra 2012 p.169).
Profiles of Milford Sound during the Late LGIT (Last Glacial-Interglacial Transition)and the Holocene (Dykstra, 2012 p.195) showing impact of changing sea level and disappearance of glacial activity.
Profiles of Milford Sound during the Late LGIT (Last Glacial-Interglacial Transition)and the Holocene (Dykstra, 2012 p.195) showing impact of changing sea level and disappearance of glacial activity.

The landslide generated tsunami threat at Milford

Dykstra's study ultimately attempts to assess the risk of a landslide generated tsunami in Milford Sound. He suggests that the risks of a big earthquake causing a 4m high tsunami are real enough to undertake proper risk assessment of such an event. In Norway in 1934 41 people were killed by just such an event in Tafjord (p.249).

 

Using the example of a Norwegian fjord that is likely to experience a large landslip-generated tsunami (the Aknes mountain at Storfjorden) Dykstra suggests the development of sophisticated mathematical modelling and physical model based simulation of landslide and tsunami effects in Milford Sound.

A fishing boat from the Deepwater Basin heading up Milford Sound past the entrance to Sinbad Gully.
A fishing boat from the Deepwater Basin heading up Milford Sound past the entrance to Sinbad Gully.

A large Alpine Fault earthquake is due - it hasn't ruptured in 295 years and the recurrence cycle is approximately 330 years (p.260).

 

Due to the short distances between tsunami generation and centres of habitation such an event would leave almost no room for warnings, particularly as it is estimated waves could travel down the Sound at speeds of up to 190km/hour.

 

A quick search of the Internet suggests that not much has been put in place to meet Dykstra's recommendations.

The Milford Mariner heading into a fierce down-Sound buffeting with the bulk of the Pallsiades on the R and vertiginous cliffs of Mitre Peak on the L.
The Milford Mariner heading into a fierce down-Sound buffeting with the bulk of the Pallsiades on the right and vertiginous cliffs of Mitre Peak on the left.

These concerns are echoed in the Geology New Zealand report on Fiordland.

 

Tsunamis and more localised seiche are a high risk in Fiordland. 'A major rock fall into any Fiordland lake could have potentially disastrous results for lakeshore constructions and their occupants.' (A seiche is a standing wave oscillating in a body of water - water sloshing in a bucket is a mini-seiche - National Ocean Service.)

 

Similarly underwater earthquakes along the Puysegur subduction zone could trigger rapid 4m-high tsunamis. Catastrophic damage would only be likely  where the fiord coast is settled - places like Milford Sound, West Arm and Deep Cove (Turnbull et al. 2010 Geology of the Fiordland Area, GNS Science pp.83-4)

The entrance to Milford Sound  in heavy cloud and early morning light. Not a good place to be caught by an earthquake generated landslip tsunami of 4 metres traveling at 190km/hour.
The entrance to Milford Sound in heavy cloud and early morning light. Not a good place to be caught by an earthquake generated landslip tsunami of 4 metres travelling at 190km/hour.
Our bed for the night, the Milford Mariner, moored in Harrison Cove with the imposing Mt Pembroke and cirque glacier towering above in Milford Sound.
Our bed for the night, the Milford Mariner, moored in Harrison Cove with the imposing Mt Pembroke and cirque glacier towering above in Milford Sound.

Deepwater Emergence

The fiords of Fiordland were carved out  some 20,000 years ago. As the ice melted and the glaciers warmed at their seaward ends they deposited vast quantities of moraine at the entrance of each fiord. These terminal moraines have formed a partial barrier to the ingress of saltwater. In most of the thirty fiords in Fiordland the circulation of water is confined to the top 20–40 metres. Below that the water can remain undisturbed for years.

 

Rainfall (up to 7.5 metres annually) is stained by tannins as it passes through the hanging forests and rotting leaf litter until it is the colour of strong tea. This discoloured and opaque freshwater forms a layer on top of the saltwater of the fiords.

Kayaks  over the Stirling Basin emphasise the immensity of Milford Sound.
Kayaks over the Stirling Basin emphasise the immensity of Milford Sound.

At times of heavy rain this can be 30ft deep.  The discoloured water filters out light and constrains the growth of seaweeds. This allows encrusting and sessile animals like sponges and corals to flourish on the underwater walls.


These would normally live at deeper depths to thrive. One of these are black coral trees. There are reckoned to be some seven million colonies in the Fiordland area and some are 200 years old. Others are gorgonian fans and sea pens—strange, quill-shaped corals.

 

This is a phenomenon called ‘deep water emergence’.

Sindbad Gully shrouded in cloud. It is also home to the endangered Sinbad Skink.
Sindbad Gully shrouded in cloud. It is also home to the endangered Sinbad Skink. Below the 300m depths of the Stirling Basin.

'In the lower zone (15–40 metres) large sponges, sea squirts, corals, hydrocorals and lampshells (brachiopods) dominate the gloomy rock walls. Black coral grows in abundance in colonies up to 5 metres tall. Two sea cucumbers, the white (Ocnus species) and strawberry (Ocnus brevidentis) are found although the former is more common than the latter. Lampshells are ubiquitous and in some areas it is they can reach densities of 1,000 per square metre.'

 

'The large deep ocean and polar glass sponge (Symplectella rowei) is found at depths of 30–50 metres. Below this life thins out dramatically to 450 metres. Heart urchins and tube worms predominate to depths of 200 metres; below this, shellfish, heart urchins and crabs live in a muddy ooze like that at depths of 1,000 metres in the open ocean.'

 

(See Te Ara: Fiords).

Red seeweed and barnacles at the highwater mark on the sidee of Milford Sound. Little can grow in the freshwater that overtops the saltwater in the Sound.
Red seeweed and barnacles at the highwater mark on the sidee of Milford Sound. Little can grow in the freshwater that overtops the saltwater in the Sound.
The alluvial fan in Harrison Cove. Rates of erosion at Milford Sound are eextremely slow compared to other fiord regions of the world. Hard rock, thick vegetation, small catchments and sediment filter
The alluvial fan in Harrison Cove. Rates of erosion at Milford Sound are extremely slow compared to other fiord regions of the world. Hard rock, thick vegetation, small catchments and sediment-filtering lakes all keep the sediment load into the fiord low.

Life in the fiords is governed by light, depth, the salinity of the water and the availability of rock walls.

 

'In the upper zone between the surface and 15 metres fresh water restricts marine life to that which is capable of living in brackish water down to 5 metres. This impoverished zone is dominated by green seaweeds (Ulva species), mussels, barnacles (Eliminius species), shrimps (Palaemon affinis) and the small cushion star Patiriella regularis.'

 

'Between depths of 5 and 15 metres the rock walls are encrusted with a diverse community of tube worms, sponges, soft corals, sea squirts and molluscs. A variety of starfish, urchins, sea snails and sea slugs predate on these. If rainfall is low and the freshwater layer thins big big 11-armed starfish (Coscinasterias calamaria) move up the rock walls to feast on mussels.'

A skein of rain forest clings to the cracks and ledges created by the passing of numerous editions of the Milford Sound glacier.
A skein of rain forest clings to the cracks and ledges created by the passing of numerous editions of the Milford Sound glacier.

There are over 150 species of fish in the fiords.

 

A species of brotula, a muddy brown, pin tailed, live-bearing fish has been named 'Fiordichthys slartibartfasti' in a  nod to Douglas Adams’ 1978 radio series, The Hitchhiker’s Guide to the Galaxy.

 

Blue cod (Parapercis colias) and other fish have in many parts not learned to fear humans and will swim up to divers. The Jock Stewart/Red Gurnard Perch (Helicolenus percoides) and scorpion fish (Scorpaena papillosa) hang around on the bottom of the fiords occasionally in black coral trees.

 

The former is good to eat and caught as a bycatch in some fisheries or through bottom longlining (see Forest and Bird).

 

Quite why the Red Gurnard Perch is known as the 'Jock Stewart' fish is not clear. Robert Leslie Stewart was one of the last executioners in the UK in the 1950s and known as 'Jock Stewart'. Perhaps the bulging eyes of the perch are supposed to resemble the hangman's victims?

 

The 2,000 ft (600m) vertical cliff face of The Lion (1,302m) in Milford Sound.
The 2,000 ft (600m) vertical cliff face of The Lion (1,302m) in Milford Sound.

Conservation of the unique mix of marine life in the fiords has been hampered because the jurisdiction of the national park ends at the highwater mark. Threats come from commercial fishing in the fiords and growing pressures from diving and tourism.

 

Small reserves have been established and a group, The Guardians of Fiordland Fisheries and Marine Environment, was formed in 1995.

 

Made up of local people, the fishing industry and charter operators the group has been instrumental in creating the Sutherland Sound Marine Reserve, (recognised in the Fiordland Marine Management Act 2005), which ensures that 928,000 hectares were set aside as a special management area, and created eight new marine reserves.

 

It turns out the Sutherland Sound/Te Hapua Reserve is in a fiord with an entrance shelf that is too narrow to cross safely in a boat. To my cynical eye it's designation as a reserve of 449 hectares looks opportunistic as it was rarely visited, little studied and not a source of commercial fishing.

Mt Sheerdown, Odyssey and Ada and the spur of Mt Phillips in Milford Soun
Mt Sheerdown, Odyssey and Ada and the spur of Mt Phillips in Milford Sound

There is concern about the killing the golden Milford Sound goose but thus far it seems to be outweighed by commercial imperatives.

 

A report on 'The effects of commercial sea-surface activity in Milford Sound' was produced for Environment Southland by the University of Otago in 2006.  It calls for a fuller research programme whilst noting some concerns to do with black coral die-back and sewage disposal, ship safety and the general pressure of numbers on Milford Sound.

 

Milford Sound: history

 

for more on geology and glaciation see

 

New Zealand: the Southern Alps

 

and

 

Arctic Norway: the Lyngen Alps