Wedge Fifty: The Catskills Conundrum

The following mystery was written for Accretionary Wedge #50, hosted by Evelyn of Georneys. This month we are invited to:

Share a fun moment from geology field camp or a geology field trip. You can share a story, a picture, a song, a slogan, a page from your field notebook– anything you like!

On to the story…

The Brunton Compass is a field geology staple. Image from the Brunton website (click to visit).

Every geologist worth their rock salt recognizes – and hopefully knows how to use – a Brunton Compass (Evelyn gave them their due in B is for Brunton). Housing a compass (with adjustable declination), clinometer and mirror at less than 10 ounces, the Brunton is important as much for its form as its function. One of the more common uses of a Brunton is to take strike and dip measurements of strata. Strike indicates the compass direction of the originally horizontal bedding plane (i.e. the orientation). Dip is the angle relative to horizontal in the downward direction of the bedding plane, measured with a simple adjustable bubble level.

Visualizing strike and dip can be tough at first, and it’s easier done than said. That’s why, on an undergraduate class trip to the Catskills, our first task of the day was to warm up with some Brunton practice – and we needed some warming up. It was crisp late September morning, and many of us were sporting geology club hoodies, warm hats and gloves. Fortunately the Catskills record a smorgasbord of interesting geologic events to get the blood flowing, and our first stop of the day was a doozy.

The “Taconic Unconformity” near Catskill, NY. Steeply dipping Ordovician sandstone interbedded with shale (right) lies unconformibly below a not-quite-as-steeply dipping Silurian-age limestone and medium-grained sandstone (left).

There is no evidence of deformation on the discontinuity (it is an angular unconformity), but there is a fault zone as well, with slickenlines. Ron Schott’s gigapan of the area shows the broader context, though I couldn’t find any slickenlines. Anyway, Bruntons in hand, we spread out over the outcrop to measure the strike and dip of the surfaces with slickenlines. Some of the not-quite-awake students worked in pairs.

It is not difficult to persuade geologists to climb. Here the structures class swarms an outcrop in the Catskills to practice using a Brunton in taking strike and dip measurements.

After our Professor had made the rounds to see everyone had the general idea, we collected together and reached a general consensus of strike and dip measurements. The slickenlines were striking towards the west-northwest and were dipping around 45 degrees south…I think. My notes from this trip aren’t very good, but everyone seemed to be in agreement. Well, almost everyone.

One student spoke up about some wonky strike measurements she had recorded. Sometimes they would be striking west, but then other areas seemed to say the slickenlines were striking north or southwest. Her dip measurements were spot on, but she was getting no consistency with strike. It wasn’t a method issue, as she demonstrated she used the normal strike-taking steps. We had a mystery on our hands! A nice little brain teaser to start the morning. The Prof started running through a process of elimination to find the source of error.

It is possible for magnetic minerals in rocks to mess with the compass and give erroneous strike measurements, but that was ruled out as the rest of us were getting consistent results. The Prof took strike in one area, then had her measure the same location and it was way off. They swapped Bruntons with the same result. A little frustrated, the student took off her fingerless mittens to get a better feel when taking strike measurements. She remeasured the strike and finally read a west-northwest strike. The Prof gave her back her Brunton, and the needle once again pointed west-northwest. Things seemed back to normal…but what caused the slew of mis-measurements that morning?

The Prof figured it out first. He asked to see a mitten, which had a flap that could be folded back to make a fingerless glove. The student had been using it in the fingerless configuration, with the flap held securely to the back of the mitten. The Prof folded the flap near the back of the mitten and smiled as several small but powerful hidden magnets pulled the flap back into place with a dull thud.


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