Think of a glacier as a frozen river flowing slowly down the mountain (figs. 13-1 and 13-2). When the slope steepens, the glacier flows faster.
Just as rivers form rapids, glaciers form crescentic crevasses when ice on steeper pitches flows faster and breaks away from slower moving ice above.
SNOW AND NEVE
SNOW BRIDGE 1
CREVASSES / NUNATAK
Fig. 13-1. Cross-section of a glacier showing principal features
3. Crescentic crevasses
4. Firn line
6. Marginal crevasses
7. En echelon crevasses
8. Terminus or snout
9. Braided outwash stream
10. Outwash plain (and ground moraine)
12. Old lateral moraine
13. Moraine lake
14. Terminal moraine
15. Old terminal moraine
16. Medial moraine
17. Lateral moraine
Fig. 13-2. Aerial view of a glacier showing principal features
The crevasses form at a right angle to the direction of flow.
In a river, a steep drop brings a waterfall; the glacial counterpart is an icefall. Crevasses shoot out in all directions from the base of an icefall, and large ice towers (seracs) hang above. Gradually, gravity combined with the forward movement of the glacier will bring these seracs crashing down.
Below steep sections, the glacier slows again, and there may be no crevasses at this point. But if the decrease in slope is too sudden, the glacier may buckle, creating broken, uplifted waves (pressure ridges) as fast-moving ice rams slower ice.
The glacier moves faster in the center than along the sides, where friction along the walls of its route slows it down. This difference in speed often creates lateral (marginal) crevasses, which trend up-valley, near the sides of the glacier. Both lateral and crescentic crevasses tend to angle upstream toward the glacier center, a good clue in guessing the hidden extension of a crevasse from a single hole on the surface. (Any series of crevasses that are parallel to each other are known as en echelon crevasses.)
Unfortunately, minor irregularities underneath a glacier can produce random fractures with no pattern. For example, protuberances of rock under the ice, called nunataks, usually form a halo of crevasses. If the rock doesn't actually reach the
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