Erosion by Alpine Glaciers


Alpine glaciers originate high up on a mountain. The glacier sitting on top of the mountain erodes back into the mountain top. This erosion carves a bowl shaped valley head into the peak. This steep, blunt "bite" taken out of the mountain is a CIRQUE. After the ice melts, a depression in the bottom of it may hold water to form a lake called a TARN. If the glacier grows larger, it will extend down a valley carving into the sides and deepening the valley into a classic U-shaped cross section. These U-SHAPED VALLEYS are also called TROUGHS. Low places carved out of the rock beneath the glacier may retain water and form TROUGH LAKES. Different types of trough lakes include FINGER LAKES with their long, narrow shapes and PATERNOSTER LAKES, a chain of small round trough lakes. If the glacier reaches the sea, it extends out into the water while it is still downcutting and eroding the original stream valley deeper. When the ice melts, the sea replaces the ice flooding up into this deepened valley creating a FJORD (also fiord). A fiord, then, is a sea-flooded glacial trough or U-shaped valley. Two parallel adjacent valleys may become glaciated. As the glaciers erode their U-shaped valleys into the prior stream valleys, they narrow the land in-between. That divide can become a very narrow, jagged, saw-tooth like ridge identified as an ARETE. A low saddle shaped area on such a ridge, or where two cirques eroding back meet creates a pass called a COL. Three glaciers eroding cirques into a mountain can carve it into a very steep pyramid shaped feature called a HORN.

The last two landforms associated with erosion by alpine glaciers are HANGING TROUGH and HANGING VALLEY. The distinction between these is often INCOMPLETELY OR IMPROPERLY DONE in many texts. The distinction may not be made in the lecture section; however, for lab we will distinguish between the two. Imagine tributary stream valleys in a mountainous area. The climate becomes colder and glaciers form and move down the valleys. The glacier in the main stream valley will be larger than the glacier in the tributary valley. The two glaciers meet and join. The main glacier is bigger and more powerful, so it will cut its valley deeper than the tributary glacier. Both are forming U-shaped valleys. When the glaciers melt, the bottom of the valley of the tributary glacier will be higher than the valley bottom of the main glacier. The tributary valley will be "hanging" on the side of the main valley. A stream flowing in it will join the stream in the main valley by creating a water fall at the edge of the main trough. The feature that has formed here is a HANGING TROUGH. Both valleys were glaciated.

Now, return to imaging the original two streams. If the main valley becomes glaciated, but the smaller, tributary stream remains unglaciated the main valley will be eroded by the glacier much deeper than the bottom of the stream is able to cut. When that glacier melts, the tributary stream valley will still have its youthful shape, a V-shaped cross section. The main valley, though, having been glaciated will have the U-shaped cross section. The tributary valley is a HANGING VALLEY. It is a V-shaped fluvial valley "hanging" on the side of a glacial trough.


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