TIL a compass can be fooled by a single cliff face in the Tetons

Last month near Jenny Lake I followed a faint route marker straight into a granite wall for 40 minutes before realizing the iron in the rock had my needle pointing sideways, has anyone else had their bearings scrambled by geology like that?

2 comments

2 Comments

the_william1mo ago

Magnetic rocks mess with compasses, sure, but the Tetons are mostly granite and gneiss. Not a lot of iron in that stuff. More likely you had a bad bearing from the start or hit a local anomaly from a different source. I've hiked around Jenny Lake a dozen times and never seen the needle swing more than a couple degrees. Forty minutes off route sounds like a map reading problem, not geology. Happens to everyone once or twice.

tylerj221mo ago

Three degrees is still a real swing though, and I've seen the needle do weirder stuff near the moraines on the west side, not around Jenny Lake. That's where the glacial till gets mixed with all sorts of stuff, including chunks of iron-bearing rock that came from way up in the range. So what if the map was right, but the person was standing on a pile of that till when they took their first bearing? Wouldn't that throw you off right from the start, and then you'd just be walking a straight line that's already wrong? I'm not saying it's for sure, but I'd want to know if they checked their compass in a few different spots before committing to that route. That would tell us a lot about whether it was the rock or the person.