|
Lake Te Anau
Lake Te Anau is in the southwestern corner of the South Island of New Zealand. Its name was originally Te Ana-au, Maori for 'The cave of swirling water'. The lake covers an area of 344 km², making it the second-largest lake by surface area in New Zealand (after Lake Taupo) and the largest in the South Island. Lake Te Anau is however the largest lake in Australasia by fresh water volume.
The main body of the lake runs north-south, and is 65 km in length. Three large fiords form arms to the lake on its western flank: North Fiord, Middle Fiord and South Fiord. These are the only inland Fiords that New Zealand has, the other 14 are out on the coast. Several small islands lie in the entrances to Middle Fiord, which forks partway along its length into northwest and southwest arms, and South Fiord. The lake lies at an altitude of 210 m, and since its maximum depth is 417m much of its bed lies below sea level.
Several rivers feed the lake, of which the most important is the Eglinton River, which joins the lake from the east, opposite the entrance to North Fiord. The outflow is the Waiau River, which flows south for several kilometres into Lake Manapouri. The town of Te Anau lies at the south-eastern corner of the lake, close to the outflow.
Most of the lake is within Fiordland National Park and the Te Wahipounamu World Heritage site. Other than the Te Anau township, the only human habitation close to the lake is the farming settlement of Te Anau Downs, close to the mouth of the Eglinton River. Between these two settlements the land is rolling hill country, but elsewhere the land is mountainous, especially along its western shore, where the Kepler and Murchison Mountains rise 1,400 m above the surface of the lake.
The Te Anau region was shaped by, amongst other forces, enormous and powerful glaciers, forming the valleys of the North, Middle and South Fiords, and flowing in a generally southwest direction down the lake. All of the ice-sculpted landforms of the Southern Alps are the product of advances and retreats of the ice in the South Island during the last 250,000 years. The ice covering the Te Anau region was up to 2km thick.
The most extensive moraines are from the most recent glaciation, the Otira Glaciation, which reached its maximum around 18,000 years ago. As ice has retreated, the depressions behind some of these moraines have filled with water, creating some of New Zealand’s most scenic lakes, such as Te Anau, Wakatipu, Tekapo and Pukaki.
Roche Moutonnée
In glaciology, a roche moutonnée (or sheepback) is a rock formation created by the passing of a glacier. When a glacier erodes down to bedrock, it can form tear-drop shaped hills that taper in the up-ice direction.
The 18th-century Alpine explorer Horace-Bénédict de Saussure coined the term 'roches moutonnées' in 1786. He saw in these rocks a resemblance to the wigs that were fashionable amongst French gentry in his era and which were smoothed over with mutton fat (hence 'moutonnée') so as to keep the hair in place.
The appearance of the erosional stoss and lee feature is very defined on Roche moutonnée as all the sides and edges have been smoothed and eroded in the direction that the glacier that once passed over it. It is often marked with glacial striations.
The rough and craggy down-ice side is formed by "plucking", the erosional process in which ice melts slightly by pressure and seeps into cracks in the rock. When the water refreezes, the rock becomes attached to the glacier. But as the glacier continues its forward progress it subjects the stone to frost shattering ripping strips away from the rock formation.
Note that the side profile of a roche moutonnée is opposite to that of a drumlin. In a drumlin, the steep side is facing the approaching glacier, rather than trailing it.
Drumlin
A drumlin – from the Gaelic word droimnín ("little ridge"), first recorded in 1833 – is an elongated whale-shaped hill formed by glacial ice acting on underlying unconsolidated till or ground moraine.
There are many theories as to the exact mode of origin and plenty of controversy among geologists interested in geomorphology. Some consider them a direct formation of the ice, while a theory proposed since the 1980s by John Shaw and others postulates creation by a catastrophic flooding release of highly pressurized water flowing underneath the glacial ice. Either way, they are thought to be a waveform (similar to ripples of sand at the bottom of a stream).
It is not clearly understood whether drumlins are erosional or depositional features. It is also poorly understood why drumlins form in some glaciated areas and not in others. They are often associated with ribbed moraines.
Actively forming drumlins have been reported from Iceland and Antarctica; in the Icelandic glacier field, they are formed by the sculpting of previous deposits by cycles of erosion and deposition as the glacier surges forwards then retreats. Erosion can be on the order of a meter's depth of sediment per year, with the eroded sediment forming a drumlin as it is deposited.
The long axis of a drumlin is parallel with the movement of the ice, with the blunter end facing into the glacial movement. Drumlins are typically 1 to 2 km (0.6 to 1.2 mi) long, less than 50 m (165 ft) high and between 300 to 600 m (~0.25 mi) wide. Drumlins generally have a consistent ratio of 2:3.5 width to length dimensions. Drumlins are often in drumlin fields of similarly shaped, sized and oriented hills.
Drumlins usually have layers indicating that the material was repeatedly added to a core, which may be of rock or glacial till. The composition of drumlins varies depending on the area in which they are found, and can consist of similar material to the till of the surrounding moraine or be composed almost entirely of bedrock, sand and gravel or various mixtures thereof.
This Earthcache
In order to log this earthcache, you will need to visit the Dome Islands around the published coordinates and perform the tasks listed below. Answers are to be emailed to the cache owner, and not included in your online log.
1. Describe the general shape of the islands in terms of profile and plan view, with respect to the direction of the glacial ice flow.
2. Based on what you have read and learnt about roche moutonée and drumlin glacial formations, which would the Dome Islands be classified as? Justify your answer.
3. Describe the shoreline around the islands. Is it steep into deep water, shallow, rocky, sandy, etc?
4. Please take a photo from near the published coordinates, including your GPS unit with South Fiord (to the west) in the background. Do not include the Dome Islands themselves in your photo. You should upload this photo with your online log - don't email it to the cache owner.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
Thanks wikipedia, and other sources.
|