The more time I spend on glaciers the more I look back at my own journey and feel like I was not given a very good framework for understanding the glacier environment. Now, having spend over 600 days on the glacier I’m going to try to articulate some fundamental concepts that I find useful. It comes down to two main ideas: identifying where crevasses form and the characteristics of the snow that covers them.

Crevasses can form a lot of weird places you wouldn’t expect–always keep that in mind. But generally there are two principal types of formation. Snickers bar cracks and speed change cracks.

The classic analogy to start with is the snickers bar. When you bend it, the inside bends the outside cracks. I’ve heard from several longtime mountain guides that around 150-200 feet are the deepest a crevasse can be. Down that deep the pressure makes the ice pliable enough it no longer cracks it only flows. It’s the inside of the bar. This depth seems reasonable given my own personal experience. Although it’s quite hard to find holes this deep because they will typically begin to fill with snow before too long (but if you know where to look you can find the blackness).

You might also call this formation a tension release crack. This is the classic and most prominent mechanism of formation we see on the glacier. Cracks form around convex features and close around concave features. This phenomenon can present as a few horizontal crevasses that open less than ten feet before they close again or as a significant icefall as the cracks grow to the extent the blocks of ice in between the crevasses loose integrity and begin to erode and and collapse.

Key observations about this kind of crevasse formation is the first crack to open as the glacier enters the convexity can many times occur surprisingly far into the flat terrain above the icefall. This crack can also tend to be the most intimidating as it has recently opened and has not always had a lot of time to fill with snow. This can be the one where you look into endless blackness. This type of crevasse can run uninterrupted for the whole width of the convexity and only be crossable by true snow bridges. These cracks can form significant barriers to travel as the crossable snow bridges erode.

The other kind of prominent crevasse formation is one from changing speeds of the ice. These tend to form more as eyebrow shaped tears in the glacier. Imagine pulling super cheesy pizza slowly apart at an angle. While these cracks can be quite deep they are many times more manageable because there will be many ways through them on top of the tongues of ice that form between the eyebrows and provide safe pathways though. While at casual glance it might seem like a snow bridge they are not and maintain structural integrity long into the melty season.

These crevasses tend to form along the edges of the main glacial flow as the speed slows near the margins. Or where two glaciers meet or diverge. The intersection of glaciers can produce some of the more complex crevasses and challenging navigation I’ve encountered.

The relationship between the snow environment and glacier environment

Identifying where crevasses are prone to forming is a great starting point, but ultimately it is the snow that obscures the glacier and what creates the uncertainty. Once the snow has melted from the glacier there is no more uncertainty, what you see is what you get. While this “dry glacier” is beautiful and in some ways less hazardous, it is many times much more challenging to travel on and impossible to ski on. It will generally be hard on the knees and ankles as you walk on hard glacial ice in crampons. Even mild slopes can become hazardous when ice where they would have drawn little consideration while snow covered.

As you move lower on the glacier, down into the ablation zone, the makeup of the glacial ice itself begins to change from the higher accumulation zone. The lower glacier will loose its seasonal snow cover completely and the ice will begin to erode as it flows downhill. This eroding ice will still have a seasonal snow pack for on it and will still experience new cracking but takes on a different character as the higher glacier which never experience any melt driven erosion. This can create more folded and irregular layers of ice as opposed to the neat strata we find higher on the glacier.

Understanding snow deposition is they key in many ways but also very challenging. You need to be able to build a good mental picture of how the glacial environment overlaps with the seasonal snow environment. This occurs on a seasonal basis as well as an elevational basis. Gauging the uncertainty of your snow deposition picture t is key as snow cover is extremely complex and there can be a lot of spacial variability of snow deposition.

Elevation zones of the glacier

At the start of the glacier there is the accumulation zone, at the end is the ablation zone. Late in the summer season, the accumulation zone is the areas that still have snow. These areas will add more ice to the glacier as they are buried by subsequent seasons snow. The ablation zone will present as the areas that are dry glacier, where the glacier ice is melting and loosing mass. In between is called the firn line. In the higher reaches of the glacier we tend to have a much deeper snowpack that effectively insulates us from many of the crevasses. In lower elevations the seasonal snowpack is thinner and we are far more exposed.

In broad strokes: the higher elevations tend to be more forgiving as they are deeply buried and the super low elevations are more forgiving as they are dry glacier and the cracks are obvious. It’s the middle elevations that are tricky–areas that have big cracks and snow that is no longer effectively bridging them. The areas where this “zone of concerning coverage” are throughout the course of each snow season is what we need to be tuned into.

Big picture we need to think about how fast the crevasses are opening and is the snow climate in that part of the season and elevation on the glacier one that effectively bury those crevasses relative to the rate they open.

In the higher reaches of the glacier, in an ideal world, we have consistent snow though the winter that will bury the glacier deeply. This winter snowpack forms a fairly homogenous layer of snow across the glacier that becomes very strong. As the seasonal snow blanket is growing the crevasses are opening at a relatively slow rate, making for nice strong snow bridges.

As the snowfall slows into summer and we get a lot more heat, the snowpack begins to settle and transition. Snow bridges begin to sag as the melt form transition takes hold, and cracks form in the winter snowpack as the glacier flows. We begin to see many clues on the snow surface as to where crevasses are. In an ideal world, as the winter snowpack melts and the crevasses become exposed and more hazardous we get plenty of clues to their presence.

Generally speaking, with relatively continuous winter storms we have well covered glacier and a less concerning environment. Once into summer and a relatively long period of heat and melt form transition we have plenty of clues to the presence of crevasses and find a less concerning environment.

The concerning glacial environment

Circumstances that produce a scary glacial environment generally involve the combination of open crevasses and snow characteristics that can obscure clues to their presence. Having a gauge of the weather history prior to getting out on the glacier is key as recent snow and especially lots of wind is one of the biggest red flags. When this overlaps with a lot of open holes we have a problem.

During the hight of the deposition season and in the accumulation zone, recent snow and wind isn’t as big of a deal because the coverage is good–the rate of glacial flow and deposition are burying the holes effectively. But in lower elevations where the coverage isn’t as good, or in later season conditions where more holes are open the isolated storm creates a period of extremely hazardous conditions.

This spring on Mt Baker we found some classically hazardous conditions. There had been a period of summer weather and some holes had started to peak through the winter snowpack. Many were fairly thin and very deep, not having had a chance to fill in with snow at all. A subsequent period of winter like weather buried these holes with a veneer of snow that obscured all clues to their presence but provided no protection.

When the snow and glacier environment gets out of synch.

Generally speaking, in the accumulation zone–if the snow environment and the glacier environment are in synch–the glacier is well covered by the winter snowpack. The seasonal snow environment determines the size and speed of the glacier and the faster the glacier the deeper the snow environment. This becomes a concern as we see glaciers built by snow climates with far more snow receive far less seasonal snow or far less seasonal snow accumulation due to melt. If we have a fast flowing glacier built from a historically high snow environment which is no longer blanketed well because of chronically low snow totals we have a scary setup. This has resulted in lots of accidents during low snow seasons in parts of the Apls where we have a lot of people in environments with larger fast flowing valley glaciers.

Notes on limited flow pocket glaciers

Another product of climate change is the dying glacier. These pocket glaciers–the receding remnants of larger glaciers either flow extremely slowly or not at all. They do still have crevasses though. This creates a dangerous situation, because as the seasonal snowpack thins over these crevasses there is no flow causing them to provide clues to the crevasses presence.

It’s very possible for bridges to be only 8 inches think and easily breakable but with very limited clues to their presence. It is almost closer to falling into a creek in a more maritime snow climate. I have had some scary near misses as well as heard of some in areas with small pocket glaciers that you would not expect the glacial hazard to be very elevated.

When the snow is less than around 8-10 inches over an open crevasse or a creek the snow surface tends to present with a slightly different texture. Its possible to identify this situation by subtle observation of the surface and probing with a pole.

Concluding thoughts

This is meant to be a brief introduction to some glacier and snowpack concepts I would suggest pursuing some level of mentorship when it comes to glacier travel. Especially on skis. By its nature glacier skiing is usually an unroped endeavor and the margin for error is small. You cannot learn about snow bridges by direct experience when traveling on skis. You need indirect experience (mentorship).

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