This video is part four of a four part series.
Glacial erosion carves out a variety of landforms. Even in regions where there are no glaciers, many landforms are evidence they once existed there. The South Island has hundreds of fiords, 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 point of a glacier. Corries have a rounded hollow with a steep back wall.
Corries 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 bergschrund's 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.
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 Arêtes.
Sometime three corries back into each other. They eventually erode backwards to leave a sharp pyramid peak.
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.
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 Fiord. 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 a 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.
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.
This video was made in New Zealand.
Satellite images in the video are provided by Google and its map data providers.
Satellite images of Milford Sound: 2020 Maxar Technologies
Satellite images of Norway and its fiords: Image IBCAO, Image Landsat/ Copernicus, Data SIO, NOAA, US Navy, NGA, GEBCO, Image US Geological Survey
Satellite images of Canada's west coast: Data SIO, NOAA, US Navy, NGA, GEBCO, Data LDEO Columbia, NSF, NOAA, Image Landsat/ Copernicus, Data LDEO Columbia, NSF, NOAA
Satellite image of Fiordland: Data SIO, NOAA, US Navy, NGA, GEBCO, Image Landsat/ Copernicus, Data LDEO Columbia, NSF, NOAA