Strike, Dip, and Skis

Hoodoo summit

Mt. Washington selfie on top of Hoodoo Butte!

It’s been something of a theme in my recent blog posts that the west-coast mountains are a source of nerdy joy for this recently transplanted geoscientist. This winter I went on my department’s grad student ski weekend to try out this whole zooming-down-mountains-on-sticks thing and get a chance to hang out at Mt. Bachelor with a convivial bunch of folks. I got thoroughly hooked despite spending much of my time crashing on the bunny hill and watching the 4-year-olds effortlessly ski past me. It’s a whole new way to appreciate the volcanic history of the northwest! (hmm, that sounds like a blog post series once this term is finished…)

Two weekends ago I progressed to wiping out on the blue-level slopes instead of the greens (baby steps!), and a thought hit me out of the blue that dramatically improved my control over my skiing.

‘What would a Brunton compass tell me about this line?”

I promise I hadn’t hit my head and lost my marbles on the previous “yard sale” fall where my scarf, poles, and skis ended up strewn around me. For my non-geologist readers, a Brunton compass is the strange but useful combination of a compass, mirror, and level that geologists use to measure the dip (steepest angle) of a tilted piece of rock as well as the strike (cardinal direction perpendicular to dip, and the direction where the Brunton is completely level).

I was skiing down the Glade route at Timberline and my trusty “aim to the edge of the run to slow down” method was failing me miserably half of the time. I had a flashback to the last time I was in mountains, swearing at my Brunton at Indiana University’s field camp in the mountains of Montana. In that moment of clarity I realized that if the mountainside was a tilted rock layer I was expecting the route to be perpendicular to the strike (true dip, i.e. straight down) while it was actually at an angle (following a shallower apparent dip). By aiming to the edge I was going to the steeper true dip and accelerating – exactly the opposite of what I wanted to to do!

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It’s a universal truth in physics, particularly noticeable for people who ski or mountain bike, that the steeper your trajectory the faster you’ll accelerate. Friction determines the maximum speed. In my case gravity conspires with my properly waxed skis to set my maximum speed on the steeper blue routes much faster than I have the skill to control, hence the wipe-outs.

If the route is straight downhill following the “true dip” of the geologic example, I can ski either to the right or left to decelerate to a more comfortable speed. This holds true for the “Over Easy” route I first skied/slid down at Mt. Bachelor, most routes at Mt. Hoodoo, or for the Magic Mile routes at Timberline. These routes are aimed pretty much straight down the side of the mountain.

true dip skiier

However if the route is at an angle to the steepest possible line, then it is following the geologic “apparent dip” of the landscape. In that case, if I ski to my right, away from the angle of the route, I would find myself skiing at a steeper angle towards the true dip and accelerating into a snow bank.

apparent dip skiier

Geology students find ourselves tripped up in less spectacular ways when faced with eroded inclined structures, where the true dip of the rock bed isn’t perpendicular to the eroded top of the formation.

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Week 5 of field camp: why the heck is the hillside shaped like an “M”?

On a contorted landscape like these hills near Doherty Mountain in Montana, it was all too easy to ignore the wobbling level bubble in my Brunton compass in the rush to finish surveying an area. After the first day we all put our notes together and realized that none of us agreed on the dip of the hillside. We had been fooled into interpreting the easiest path to walk across the hill as “horizontal” and measured various apparent dips.

apparent dip geologist

As enjoyable as my class made it to trek up those hills through the bushes and snakes, wouldn’t it be even more fun to skiĀ  down them over a nice smooth blanket of snow…

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