Glacial erosion carves out a variety of landforms. Even in regions
where there are no glaciers many landforms are evidence
that they once existed there. The South Island of New Zealand has hundreds of fjords, ribbon lakes
and u-shaped valleys that exist as reminders of the giant glaciers that once were there.
A corrie also known as a cirque is an erosion feature formed at the starting point of a glacier.
Corries have a rounded hollow with a steep back wall. Corrries form when
snow accumulates in hollows on mountainsides, slowly accumulating enough to turn 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 a bergscrund
allows water to flow onto 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 ice 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 both erode backwards until they create
a narrow knife edge ridge between them. These sharp ridges are called an aretes.
Sometimes three corries back into each other. They eventually erode backwards to leave a sharp
pyramid peak. An example of 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 glacier melts it 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 small glaciers on the summer of Mount Ruapehu.
For example, the Whakapapa glacier seen here. However at Mount Tongariro you can see the
evidence that larger glaciers once existed here in the form of the u-shaped Mangatopopo 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 also 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 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 sides may also have truncated spurs which are the ends of sloping ridges 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 to 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.
For example the 42 kilometer Lake Wanaka which lies in an ancient u-shaped valley.
The ancient terminal moraine encircles the lower end of the lake.
A smaller example of a ribbon lake is Lake Alexdrina in the Mackenzie basin.
This lake has been enclosed behind moraines left behind by an ancient glacial advance.