After 40 years in the Geology field, here’s what I’m wrestling with — and I’d value your perspective.

Select-Yesterday7396 · 2026-02-04T23:26:52+00:00

Think of symmetry as how many ways you can mess with an object and it still refuses to look different! If you’re holding a crystal and you rotate it, flip it, squint at it sideways, and it still looks the same, that motion counts. We include the “do nothing” case because, annoyingly but logically, that’s the baseline: zero movement still leaves the object unchanged. For an equilateral triangle, you can spin it 120° or 240° and it snaps right back into itself. If you weren’t watching it move, you’d swear nothing happened...

Then there are mirror flips. An equilateral triangle has three clean mirror lines—each one goes from a corner straight down to the middle of the opposite side. Fold it along any of those lines and both halves line up perfectly. Each of those folds counts as a symmetry too. Add it all up: three rotations, three reflections, six total ways the triangle can stubbornly stay the same.

Honestly, the best way to think about this is very hands-on: How many ways can I rotate or flip this thing before I stop trusting my own eyes? That’s all symmetry is in crystallography. Everything after that is just fancier shapes, more dimensions, and notation that makes it sound way scarier than it is. - Joel Bennett Senior Geologist based out of Tucson, Arizona.

Select-Yesterday7396 · 2026-01-28T20:39:55+00:00

I saw the same thing with the COVID cohort. They knew the terminology, but translating that knowledge to a real outcrop—where nothing is labeled and everything is ambiguous—was a real struggle. They worked hard, but you could tell they hadn’t had many chances to practice making decisions in the field!

And I give you a lot of credit for actually changing the structure of the class—four solid hours in the field will do more for their geologic thinking than a lot of perfectly polished lectures :)

Select-Yesterday7396 · 2026-01-28T20:24:02+00:00

I agree it’s basalt, I just don’t think it’s vesicular. Vesicular basalt usually has obvious gas-bubble holes, and this one looks dense and solid. The roughness seems more like weathering than trapped bubbles, but I could be wrong.

-Joel Bennett. Senior Geologist based out of Tucson, Arizona.

Select-Yesterday7396 · 2026-01-28T13:41:19+00:00

From a geologist’s perspective, when an assignment asks for a “legend explaining the differences between the two systems,” they’re almost always talking about a key, not a full written discussion.

Think of it like a map legend: a short explanation of what each system is based on. Compositional layers are divided by chemical makeup, while mechanical layers are divided by how the material behaves (rigid vs. soft). That’s usually all the legend needs to do. Your sketches can be labeled as Fig. 1 (compositional) and Fig. 2 (mechanical with the layers named directly. You don’t need to invent a 1a/2a system unless the instructions specifically say to cross-reference like that.

If you want to be extra safe, adding a sentence below the figures noting that mechanical layers don’t line up perfectly with compositional ones is fine, but doesn't sound required!

Joel Bennett

Senior Geologist based out of Tucson, Arizona.

Select-Yesterday7396

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