Glacial erosion carves out a variety of landforms.
Even in regions where there no
glaciers many landforms are evidence that they once existed there.
The South Island has hundreds of fjords, ribbon lakes and u-shaped valleys that exist as
reminders of the giant glaciers that were once there.
A Corrie also known as a Cirque
is an erosion feature formed at the starting points of a glacier.
Corries have a rounded hollow with a steep back wall.
Corries form when snow accumulates in hollows on mountain sides,
slowly accumulating enough to tune into ice.
This ice slowly gouges out a steep back wall through the processes of
freeze - thaw and plucking.
Large crevasses on the top of the ice called bergschrunds
allow water to flow into the ice, where it freezes to create more ice.
The bottom of the corrie is eroded by abrasion, as the ice moves in a circular motion known as rotational slip.
At the front of the ice the erosion is less, causing a lip of rock to be left there.
As more accumulates it flows over the lip and into the valley below forming a glacier.
In places where glaciers have retreated the corrie was often filled by melt-water to form a lake.
The rock lip and moraine act as a natural dam. These lakes are known as tarns.
When two corries form back-to-back they erode backwards until they create an
arrow knife-edged ridge between them. These sharp ridges are called Aretes.
Sometimes three corries back into each other.
They eventually erode backwards to leave a sharp Pyramid Peak.
An example this is Mount Aspiring in the Southern Alps of New Zealand.
Glaciers carve out u-shaped valleys just as a river produces a v-shaped valley.
Usually a glacier will follow the general direction of a river valley.
But instead of flowing around areas of hard rock like a river does,
a glacier will cut straight through it.
This means that when the way she melts leaves behind a
u-shaped valley with very steep sides.
There are numerous u-shaped valleys in the South Island of New Zealand.
The warmer North Island only has a few small glaciers
on the summit of Mount Ruapehu such as the Whakapapa glacier seen here.
However on Mount Tongariro you can see the evidence that larger glaciers once
existed here in the form of a u-shaped Mangetepopo Valley.
Some glaciers such as the Fox and Franz Josef glacier descend to low altitudes not
far from the Tasman Sea.
When global temperatures and accompanying sea levels rise,
low-lying u-shaped valleys get filled with water to become a fjord.
This produces a coastline with an irregular shape like
those found in Norway, Western Canada and Fiordland, New Zealand.
Another feature associated with valley glaciers are hanging valleys.
The floors of the tributary valleys are eroded at a slower rate than the floor of the main valley.
So the difference between the depths of the two valleys steadily increases over time.
The tributaries are left hanging high above the main valley with their
rivers and streams entering the main Valley by means of either a large
waterfall or series of smaller waterfalls.
The valley sites may also have truncated spurs which are the ends of a sloping ridge that have been cut
off or truncated by the valley glacier which flows straighter than a river would.
You can see these features in this diagram.
Finally retreating glaciers leave behind ribbon lakes on the valley floor
with the terminal moraine acting the form a dam to hold the water in.
These are also known as Finger Lakes because they are long and narrow.
The South Island has numerous lakes that fill deep valleys excavated by glaciers.
A large example is the 42 kilometer Lake Wanaka which lies in a ancient u-shaped valley.
The ancient terminal moraine encircles the lower end of the lake.