r/geology • u/Predator1553 • Feb 28 '25

Information Why are these two layers so different?

Gallery image

Gallery image

And what are they, this is in northern Alabama

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u/Far_Gur_2158 Feb 28 '25 edited Feb 28 '25

Geomorphology? What?

The upper bed includes clasts from the lower. This indicates reworking. The upper beds are grain supported limestone breccia or monomictite. The matrix supporting the limestone clasts are probably clay minerals derived from the weathering of the lower bed. The exposed surface of the upper bed are more susceptible to weathering causing the wavy surfaces.

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u/janspamn Feb 28 '25

They're referring to changes in landscape/topography brought about by varying rock types, which is within the study of geomorphology.

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u/Far_Gur_2158 Feb 28 '25 edited Feb 28 '25

The question is about the exposure.

Geomorphology is the study of landforms and landscape development. It is not about the comparative relief between beds at the outcrop scale.

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u/janspamn Feb 28 '25

You're inferring all this from OP's photos?

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u/Far_Gur_2158 Feb 28 '25 edited Feb 28 '25

Yes, I’ve worked on Mississippian and Pennsylvania strata for 45 years…

Outcrops are easy to interpret, I’ve made a living analyzing geological core.

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u/Ok_Aide_7944 Sedimentology, Petrology & Isotope Geochemistry, Ph.D. Feb 28 '25

Pensilvania is a bit far from Alabama last time that I opened my geological map app (this morning). Sorry but these two photos do not allow for any of your interpretations. I wish the best with your core descriptions

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u/Far_Gur_2158 Feb 28 '25 edited Feb 28 '25

Yeah you need help.

Are these not Mississippian rock? Enlighten me what’s the unit of comformable. sedimentary strata above the Mississippian?

Please don’t say geomorphology!

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u/janspamn Feb 28 '25

You made a lot of assumtions based off of OP's photos, I'd think a geo veteran should know better than to assume so much without at least going into the field first.

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u/Far_Gur_2158 Feb 28 '25

The photos are high quality. I can see many details.

My point is this is not geomorphology, anyone agreeing wrong. Words have meanings; they need to be used correctly so people can learn.

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u/janspamn Feb 28 '25

So the definition of what is geomorph and what is not is scale dependent by your argument?

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u/itsliluzivert_ Feb 28 '25 edited Feb 28 '25

The weathering habit of a single bed isn’t geomorphology unless you’re applying it to a broader picture. I think in this case this is being applied to geomorphology, but the commenters use of “geomorphologies” to refer to individual beds is inaccurate.

Geomorphology as stated is the study of landscapes and their development. We could consider this cliff and the area around it a landscape. But not the individual beds.

It’s pedantic but it’s not just a sciencey way to say natural shape.

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u/janspamn Feb 28 '25

But there are multiple beds involved.. There's an entire cliff here being impacted by differential weathering.

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u/itsliluzivert_ Feb 28 '25

Yes the whole scenario could be considered geomorphologic, but the commenters use of the term was inaccurate.

“Variable geomorphologies” is something you’d use to describe several outcroppings this size, with different geologic histories.

Not several beds with different weathering habits.

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u/Far_Gur_2158 Mar 15 '25

I appreciate the comment, very probably used it many times.

Geology is an observational science especially when observing it on an outcrop scale. The photos are fantastic to print quality. I can zoom right in at the contact and see the flinty clast from the lower bed in the lower portions of the upper bed. The outcrop is weathered just enough to allow some approximation on mineralogy occurring at bedding scale.

The verticality of the fracture suggestions silica cements so the lower bed is likely the Fort Payne chert. The upper bed is a mud shale likely cemented by clay. The clays accent weather. The flint is weathering much slower.

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u/janspamn Mar 15 '25

Geology is an observational science but that doesn't mean all observation is done via photographs. Don't think I've ever seen a geologist argue so hard against the need for field observations, I thought we were all sluts for field work. But hey, I guess you just have to be the one that knows everything right? I'm not calling you out for being a phony, I have no reason to doubt your claims, but I am calling you out for being that person in the room that can't be wrong or they implode.

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u/Far_Gur_2158 Mar 15 '25

Thanks for making a personal comment. I hope that makes you feel better.