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.