Fundamentals: Detecting Crevasses

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By Don Graydon.

The first step in safe glacier travel is figuring out where the crevasses are and picking a route through them. Routefinding on many glaciers is part planning, part experience, and part luck.

You can sometimes get a head start on the planning by studying photos of the glacier before the trip, because some crevasse patterns remain fairly constant from year to year. Seek out recent reports from parties who have visited the area.

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.

Guidebook photos and 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. Plan alternative routes from a distance 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 there aren?t any.

Important tips for detecting crevasses

Keep an eye out for sagging trenches in the snow that mark where gravity has pulled down on snow over a crevasse. A sagging trench on the surface of the snow is a prime characteristic of a hidden crevasse. The sags will be visible by their slight difference in sheen, texture, or color. The low-angle light of early morning and late afternoon tend to accentuate this feature. (The sags may be impossible to detect in the flat light of a fog or in the glare of a midafternoon sun, and it takes additional information to distinguish them from certain wind forms.)
Be wary after storms. New snow can fill a sagging trench and make it blend it into the surrounding surface. (At other times, however, the new snow can actually make the sagging trench more apparent by creating a hollow of new snow that contrasts with surrounding areas of old snow.)
Be especially alert in areas where you know crevasses form, such as where a glacier makes an outside turn and where slope angle increases.
Sweep your eyes to the sides of the route regularly, checking for open cracks to the left or right. Cracks could hint at the presence of crevasses extending beneath your path.
Remember that where there is one crevasse there are often many.

Snow Probing

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 ax, thrusting the shaft into the snow a couple of feet ahead of the snow you are standing on. Keep the ax perpendicular to the slope and thrust it in with a smooth motion. You need an ax 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 ax. If resistance lessens abruptly, you?ve probably found a hole. If your route must continue in the direction of this hole, use further ax 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 ax 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 ax for easier, deeper probes.