The Chill

I Fell In

There’s nothing like a clear, cold mountain stream. In Wyoming, they come straight down from patches of ice above the snow line, and they are filled with moose and brook trout. Our first two-night hike into the mountains gives us close proximity to these beautiful bodies of water. In my case, very close proximity, since I somehow manage to fall in. Luckily, my notebook and camera stay dry.

The rest of the trail takes us down a valley in between two barren slopes where we can see a different kind of alpine river. On the sides, dark stripes of volcanic rock flow down at intervals. These are dikes—volcanic intrusions into the lighter gray rocks. There are many dikes on Taylor’s map, but we’re limited by the number of samples we can carry in our packs. Taylor and Joe estimate that, between the three of us, we can carry samples from about eight sites.

When we’re travelling on foot instead of by car, we take hand samples rather than drilled cores. This means that when Taylor selects a usable rock, he has to break off a piece with a sledgehammer. Before he does, he uses the top of the orienter to measure the magnetic direction of the rock, and a compass to determine its horizontal and vertical location.

A Hand Sample

The reason we’ve hiked in to hand sample is that besides the large number of possible sites, these slopes have several places where the dikes intersect each other. These sites are especially important, because here Taylor can measure across the chill—the place where the dike cooled as it came into contact with the surrounding rock. If the surrounding rock is also a dike, it has the same high magnetic mineral content that makes it easier for Taylor to determine its magnetic direction.

Taylor Samples the Intersection

At an intersection, Taylor can see the difference in magnetization between the dike and the surrounding rock as it gets further and further from the chill. This change demonstrates that the magnetic direction of the dike is indeed different—meaning that the polarity of the rock was set by the molten dike cooling, and not by some external heating event that melted everything. If the volcanic intrusion of the dike caused the heating and cooling that set the magnetization, then the data is good, and can be used to reconstruct the ancient position of Wyoming.

A Long Way Down

In order to reach these sites, we have to leave the trail and clamber over boulders and lose stones. We’re basically mountaineering, except that our goal is not the summit but the plumbing of the mountain. There are many moss-covered rills, and Taylor says that the cracks between the dikes and the surrounding rocks often provide seep zones for tiny streams. The GPS tells us that our highest site is over ten thousand feet, which means we’re above the snow line. Our tents look like wildflowers in the valley below us.

Joe and Taylor Reach a Site

Hiking down, we can hear pika chirping in the rocks. We wait and see if they’ll appear, but they must be wary of rare human visitors.

All in all, we sample nine sites in two days—a group record. On our way out, we race a thunderstorm down the valley, our packs weighed down with dozens of helpful rocks. A cold wind pushes at our backs, but this time nobody falls in the stream.

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3 Responses to The Chill

  1. Dave Bercovici says:

    I just caught up with all your blogs Laura, and they’re fantastic. And you look like you’re having a blast, which makes me happy (I was a little worried about it). I’m sure hoofing packs of rocks isn’t that fun but it’s a good workout (day after day after day). Keep up the great work!

  2. Pablo says:

    Wow, it looks like you guys are really out in the middle of nowhere. I can’t wait to read more posts!

  3. David Evans says:

    Those above-treeline sites are a dream come true for paleomagnetism! Keep up the good work!

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