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This is another version of a May 2017 posting – enjoy again.
The west side of the Southern Alps pours rivers of ice (and water and rock) down to the narrow shelf of coastal land before draining into the Tasman Sea. It is the old dust to dust scenario, only with water. The Tasman provides the moisture that hits the mountains, climbs uphill, becomes cooler and ultimately condenses, and this process provides the moisture for the snow that eventually accumulates and becomes the ice that forms the glaciers that run back to the sea. Whew. The two glaciers that are easily reached along the South Island’s western edge are the Fox and Franz Joseph. Both of the glaciers have grown and retreated in the recent decades. Like most glaciers, they are now retreating which allows the rock debris the glaciers have gathered or that has fallen from above onto the glaciers to pretty much hide the ice itself under a layer of fractured greywacke* rock.
At Franz Jospeh you can walk for an hour or so (each way) to the toe of the glacier. The scope of the area and the walk can be understood by finding the people just a bit right of center in this image. The glacier itself is the gray stuff near the top center, not the light colored material, that’s rock. The stream bed has been filled over the recent years with broken stone washed down by the melt waters of the glacier. The surface of the glacier is now strewn with stones and there is precious little ice showing.
The river fills with stone and gravels and eventually fine silts fill in the spaces between the rocks. This makes a deep rocky river course that is surprisingly watertight on top. In Alaska it would become rich with alders but in New Zealand there are a variety of shrubs, ferns, and young Nothofagus (southern beech – though certainly not a beech). The thread of the stream will divert back and forth across the strewn rock stream bed as each channel gets blocked or when flood waters create a surge that forces the flow to deviate from the original channel. These streams are common in all glaciated terrains worldwide.
The water running out from under a glacier is not clear and certainly not blue. It is a milky gray. This color is derived from ground stone, a powder called glacial flour, the flour is created as the glacier rubs stone against stone under thousands and thousands of tons of ice as it moves downhill. The flour is very fine but will eventually settle out if the river is drained into a lake or other quiet body of water. The flour will make a soft, slippery, clay-like sediment at the bottom of the lake. This is a slow process but time pretty much runs forever does’t it.
There are times and places along the edge of a glacier where the mountain resists. The glacier is abrading, grinding the edges of the valley with the material that will (or would) become a lateral moraine; this is essentially a slurry of ground stone and ice that act like a powerful sandpaper. In other places large boulders are carried by the glacier and these boulders scrape the sides of the valley leaving lines, patterns, and shapes. These curves and grooves were carved into the rock by the movement of material contained in the edge of the glacier. The depicted area is about fifteen feet tall.
Once the glacier recedes and the surface is sealed with fine sediments vegetation moves in. In damp western South Island ferns are one of the dominant plant types. They occur quickly in recently exposed areas and are also found in the wet forest that develops on the lower mountain slopes. The non-flowering plants are well represented in this habitat with ferns, liverworts, mosses, and algae all commonly found.
Many New Zealand bird populations have been decimated by a variety of environmental intrusions; people, rodents, cats, rats, brush-tailed possums, stoats, and so on (and on and on). One of the native species that breeds well enough to maintain a population with some stability is the little Tomtit. This may look a bit like a North American chickadee or a European tit but it is a member of the Australian robin group. There are five races/sub-species/ forms/populations of the Tomtit scattered over the islands. This one was frequenting the new growth along the route of the river draining the Franz Joseph Glacier. They are birds of undeveloped areas and seem to be doing rather well in a habitat now rich in small mammals – something New Zealand never had until recently.
*Greywacke is a solidified clay, an ooze-stone of sorts. It is fine grained, gray, and reasonably homogeneous. It is sedimentary in origin and has been lifted from the Tasman Sea by the (on-going) movement of the Australian plate as it slowly crashes into the Pacific Plate. It form stone most like shale; a fragile rock that shatters and splinters easily. (The glaciation that persists in NZ is mainly along the west coast of the South Island; earth tremors can occur most anywhere.)
New Zealand sits with an island on each tectonic plate and a fracture line between the two islands. Precarious? Yes.
Surprisingly the great Alpine fault has little to do with what we have read about during the past ten years as the Christchurch area of the South Island has had several damaging earthquakes. The earthquakes that impacted Christchurch are seemingly well away from the most likely earthquake region.
this image is from Global Rumblings