shield formations

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Shield Formations

Postby Larry E. Matthews » Aug 9, 2007 9:36 am

This discussion on Shield Formations has gone on for years and years. If you go to "Cave Minerals of the World", you can find references in the Bibliography to some of the articles that have been written on the subject.

The question I raised many, many years ago is this: Is a Shield Formation determined by it's origin, or it's shape? Is there more than one process that creates formations of this shape?

If we had one isolated shield formation in a cave, I might buy the water-under-presssure-in-a-crack theory. But when there are dozens and dozens, all at different angles, that is just too bizarre to seriously believe.

Crystals growth and orientation are a much more simple explanation. And water chemistry may just be the key.

And to all of you who cave in limestone: How do you get all this water under pressure in tiny cracks, with all the big cracks available to drain it out? I certainly can't figure that theory out. At least not in the limestone I see here in Tennessee.

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Re: Shield Formations

Postby Teresa » Aug 9, 2007 9:54 pm

Larry E. Matthews wrote:This discussion on Shield Formations has gone on for years and years. If you go to "Cave Minerals of the World", you can find references in the Bibliography to some of the articles that have been written on the subject.


I don't think we're going to come up with the definitive answer with looking at the papers. You know, the biologists have their cave taxonomists. Maybe we need some geological shield taxonomists.

The question I raised many, many years ago is this: Is a Shield Formation determined by it's origin, or it's shape?


I'd be inclined to say it is both. Grabbing my big Glossary of Geology: "shield (speleo) A speleothem composed of two parallel hemicircular plates separated by a thin planar crack. Growth occurs radially along the rim where water issues under pressure from the crack. syn. palette. "

It sounds to me that there may be and probably are other rounded speleothems without the planar crack, and obviously there are a lot of cracks in caves without shields.


Is there more than one process that creates formations of this shape?

Quite likely, but they wouldn't be called shields. This devolves into a lumper or splitter nomenclature problem.

Crystals growth and orientation are a much more simple explanation. And water chemistry may just be the key.


Well. saying crystal growth orientation and water chemistry is just another way of saying pressure, since ultimately, it is the environmental conditions (including partial pressures of dissolved gas, and undissovled gaseous bubbles) which greatly affect the deposition of calcite or aragonite, or not?

Now I wonder? Are shields calcite OR aragonite?

And to all of you who cave in limestone: How do you get all this water under pressure in tiny cracks, with all the big cracks available to drain it out? I certainly can't figure that theory out. At least not in the limestone I see here in Tennessee.


I don't know Tennessee, and mostly I cave in secondary dolomite. I think of the tiny cracks as hairline fractures between the dolomite/limestone mix, as well as the differing chemistry through the rock, which yields different porosites, permeabilitities and water movement, and therefore differing solubilities. How about really really small discrete perched water sources, or distributaries along relatively insoluble, but cracked joints and bedding planes inherent in the rock?

This is proving to be an interesting discussion, IMO.
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Postby ArCaver » Aug 10, 2007 6:03 am

I wouldn't call any but the first image a shield. Those round ceiling patches of speleothem are common as dirt. They are also attached. Usually, they are a 'healed spot' where there was a previous stal fall, which took some of the ceiling with it; creating a sideways seep which eventually became filled with calcite again.


My photography leaves a lot to be desired. The formations have grown out from the ceiling several inches and during wet weather it's obvious the water is coming from the edge, not from the ceiling. Also no sign on the floor of any stal fall.

"What is an expert? A drip under pressure!" Maybe that's the answer-- shields are all experts.


I've always heard that an expert is someone who guessed the correct answer at least twice.
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A Smaple Shield Formation

Postby Larry E. Matthews » Aug 10, 2007 7:02 am

When they blasted I-24 through Cedar Ridge in Marion County, Tennessee, they created an artificial entrance to the cave shown in earlier photos in this discussion.

I picked up (about 1970 ?) a broken shield formation, sort of small (3 inches in diameter) left by the blasting, and saved it for future study.

It may be stored down at my mother's condominium. I will have to see if I can locate it. It might shed some light on this discussion.

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