The first step in staying out of crevasses is to know where they are. Sometimes you can even get a head start before the trip by studying photos of the glacier, because crevasse patterns remain fairly constant from year to year.
On the approach hike, try for a good up-valley or cross-valley look at the glacier before reaching it. You may see an obvious route that would be impossible to discover once you're there. Consider making notes or sketches to help in remembering major crevasses, landmarks, and routes.
These distant views are useful, but prepare to be surprised. What appeared to be small cracks may be gaping chasms, and major crevasses may have been hidden from the angle of your view. Flan alternative routes if you can.
Once you're on the glacier, it's a continuous game of Find the Crevasse. Just because you can't see them doesn't mean they aren't there. Here are some important tips for detecting crevasses:
• Keep an eye out for sagging trenches in the snow that mark where gravity has pulled down on snow that covers a crevasse. The sags will be visible by their slight difference in sheen, texture, or color.
• Take advantage of the low-angle light of early morning and late afternoon to spot the characteristic shadows of sagging snow trenches. They may be impossible to detect in the flat light of a fog and difficult to see in the midafternoon sun.
• Be wary after storms in fall or late spring, when new snow can mask the thin sagging roof of a crevasse.
• Be especially alert in areas where you know crevasses form, such as around nunataks, at the sides of the glacier, and where slopes steepen.
• Check regularly to the sides of your route to determine whether open cracks to your left or right could possibly extend, beneath the snow, under your path.
• Remember that where there is one crevasse there are often many.
Snow probing is the technique to use if you have found a suspicious-looking area and want to search it for crevasses. If your probe locates a crevasse, continue probing to find its true lip.
Probe with your ice axe, thrusting the shaft into the snow a couple of feet ahead of the snow you are standing on. Keep the axe perpendicular to the slope and thrust it in with a smooth motion. You need an axe with a uniform taper from the spike to the shaft, because a blunt spike or jutting ferrule makes it hard to feel the snow.
If resistance to the thrust is uniform, you have established that the snow is consistent to at least the depth of your axe. If resistance lessens abruptly, you've probably found a hole. If your route must continue in the direction of this hole, use further axe thrusts to establish its extent. The leader should open up the hole and mark it with wands.
The value of probing depends on your skill and experience at interpreting the changes felt in the snow layers. An inexperienced prober may think the shaft broke through into a hole when all it really did was hit a softer layer of snow.
The length of the ice axe becomes a limiting factor in probing. The lead climber can also carry an avalanche probe ski pole, which is lighter, longer, and thinner than the axe for easier, deeper probes.
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