How to prep your pocket fold

“Pocket folds,” as my Rockies co-instructor Pete Berquist has defined them, are rock samples exhibiting folds that are small enough to stick in your pocket (and take back to your lab). Here’s a pocket fold that I found last week in the White Mountains of New Hampshire:

I brought it home, and today I unpacked it from the car, along with about 70 pounds of other samples. I turned on the hot water tap and took out a critical piece of sample prep equipment, the wire brush (suitable for cleaning a grill), and scrubbed off all the algae. You should only use wire brushes on relatively hard samples. Because the steel wires have a hardness of 5.5, they will scratch rocks like limestone, which is made of 3.0-hardness calcite. I also gave it a quick bath in 4.5 molar HCl, to kill off any remaining algae. A rinse, and then I set it in the sun to dry.

My White Mountain pocket fold is a quartzite, with some nice graded beds, which probably show up better when they are filled in with algae. Regardless, here’s the cleaned up sample, a gray ghost of a pocket fold:

If you collect field samples for teaching purposes or just because you find them beautiful, what steps do you take to prepare them?

3 Responses

  1. Scouring powder and brush (Ajax or the like) for hard rocks, Soft Scrub (CaCO3 abrasive) & toothbrush for softer rocks and mins and for softest and friable rocks maybe just a rinse or spray with a bit of brushing to get the worst dirt and loose material off. Something I found works nice for bring out sedimentary textures and (pocket) structures generally is to cut the sample in a rock saw, then coat half of that face with lacquer or clear nail polish. I used to get a lot of funny looks when I would buy several little bottles on sale, but many more if I tries to explain they were for my rock samples. Also, as a general rule, try to get samples with faces of varying degree of weathering, and use your hammer to whack a fresh surface- different textures and structures can be highlighted on different faces. From a teaching/learning perspective, somewhat weathered faces are what you’re most likely to encounter in the real world. I can recognize a number of the bedrock types around here based on the soil color that develops on them… a very useful skill given our thick soils.

    Also, for iron staining, boiling insoluble and acid resistant samples in oxalic acid (USE PYREX) can get a surprising amount of the red and orange off, though normally not all of it.

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  3. in Ontario they blast the moss off an entire outcrop with fire hoses, scrub it with bleach, and come back the next season to map it.

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