Agates, Onyx and ancient cave questions...

[graveleye]

OK you geologists, help me out here with some questions I have. This preface might be a little lengthy, so forgvive me.

My dads house is in the southern-most reaches of TAG. There are some caves about the area including one rock shelter on his property (with alleged potential) we are going to be looking into when the tick-season is over with. Anyway, I know the entrances of some of the caves in the areas have been described as being surrounded by "agates" and I've seen them. After looking about my dads property, I've discovered that the rocks about his place are the same as I have noted in and about the entrances to some of these caves. Pretty rocks, purple, red, yellow, white all mixed together and finely striated. There is an occasional void in some of them that is filled with quartz - almost like a geode, only very tiny.
Ok... so I have a rock tumbler at home - might sound like a silly hobby, but I enjoy it. So, I brought back several pounds from my dads place, cracked them up and tumbled them and they have turned out so pretty! The more I examine these rocks, the more I realize that the striping in them seem almost to be what I might describe as the remnants of ancient cave formations like flowstone, only now they are all crumbled and on the surface. The area has quite a few caves like I said, and seems to me to be the very stumps of the mountains... perhaps the upper levels of ancient caves have long since eroded away and all thats left is these rocks. The rest of the stone in the area is more like limestone or dolomite, but his area is just chock full of these different stones. If I can get some good macro pictures, I'll post them up.

My question isnt really a question. Not being a geologist, but with those aspirations, I've found my dads land and pretty interesting study. Is it possible that the area used to be underground, and these stones really are drip and flow deposits (for lack of a technical term). I'd really like to get a good ID of these rocks if I can, and understand better how they were formed. It would be cool to tell my dad that his house is sitting where a cave used to be long long ago. (even cooler if that crack in the ground goes somewhere lol)

Anyway, if you have some theories, let me know.

[JackW]

Send me a handful of the rocks you've polished and a couple unpolished and I'll give you my best "opinion*".

PM me for my address.



* I'm not a geologist, but I'm pretty good at rock identification. Plus with your return address on the package I can look up what the geology literature says about the bed rock in your area. And as for the geologist part, I am a grad student in geology. My background is environmental chemistry.

[Teresa]

Graveleye,
you could also post some good *sharp and well-lit* photos of them on a web page and more of us could see and opine.

Agates (chert/quartz) often form in vugs in limestone or dolostone. Non-pretty rounded chert are called chert (or flint) nodules . The aren't formed by cave type deposition, but by chemical replacement of limestone around an organic or silica 'seed'. Chert also can be laid down chemically in layers, as reef structures, and as a 'fill material' such as stylolites.

We could ramble for at least an hour about chert and its chemistry and deposition. Why not put up some pics, and let us see. Often a direct scan (put the rock on a scanner glass, and a contrasting color paper behind it, not the black or white of the inside of the lid) turns out as good or better than a digicam. Correct the color as close as possible to reality. Put up photos showing three sides of the rock,, but be sure they are NOT BLURRY.

Can't wait.

[hewhocaves]

yeah, definitely put up a couple of pics.. failing that, maybe when Jack gets them he can post them or if you're going to be at OTR bring em' and I'll take photos of them.

Oh, and if you can, some sort of scale (ruler, what not) might be helpful as well.

John

[CKB69]

The Cumberland Plateau produces an amazing variety of agate,jasper,geodes,flint,and,everyones favorite,chert.

Some of this agate ranks with the worlds best,but is difficult to find.
I have some nice geodes,and,a few pieces of "Paint Rock Agate" found during many ridgewalks.
Do to the vagaries of nature,you can find nothing but porous,junk silica for miles,then,find an isolated pocket of world-class agate.

Post some picture so we can all see these beauties.

[graveleye]

well beauty is in the eyes of the beholder, although the photographer could help a little too... these might not impress, but I find them pretty. The ones I've been tumbling look even better, but they arent through yet.. still have a couple of weeks of tumbling before I can show them off.
The pictures are pretty big, so rather than hotlink them, I'll just link them from my photobucket account.

http://i14.photobucket.com/albums/a325/ ... G_0602.jpg

http://i14.photobucket.com/albums/a325/ ... G_0600.jpg

http://i14.photobucket.com/albums/a325/ ... G_0599.jpg

http://i14.photobucket.com/albums/a325/ ... G_0598.jpg

http://i14.photobucket.com/albums/a325/ ... G_0597.jpg

http://i14.photobucket.com/albums/a325/ ... G_0595.jpg

http://i14.photobucket.com/albums/a325/ ... G_0593.jpg
This one (IMG_0593) is a good example of what I was thinking looked like some sort of deposited material because of the stripes. There are lots of examples like this, but this is only one of the few I took home. I didnt find them pretty on the ground, but there is one in the tumbler now that once clean, you see that the stripes are alternating pink, white, blue in very fine lines. Cool!

http://i14.photobucket.com/albums/a325/ ... G_0592.jpg

http://i14.photobucket.com/albums/a325/ ... G_0586.jpg

http://i14.photobucket.com/albums/a325/ ... G_0589.jpg


I couldnt get any of the small quartz pockets - they were just too tiny to photograph. The batch in the tumbler are only on step 2, the second grind. These rocks are really hard compared with other rock in the area- the first grind took exactly 2 weeks to get the edges shaped. Normally with the quartz's, tigers eyes, imported and domestic agates take about a week to smooth out the rough edges and get them round, but not these. I'm not sure where they would rank on the MOHS scale though. Probably about 6-7. I rinsed them off to inspect them before starting the next step and they looked really nice. Once they get a good polish on them, they'll look great. All these specimens were on the surface, and despite the fracturing, they are not very brittle. It takes a really hard whack to get a split to happen. Sometimes they fracture like knapped flint, and sometimes they break across the flats. Although I have some arrowheads with similar patterns of blues and greys, I doubt this stone was very useful for tool making.
There are some even better specimens out in my dads pasture. It was about 98F when we pciked these up, so we werent very picky. There is one basketball sized rock that I'll be carrying home next time I go. Its completely blue, with red and white stripes in it and probably weighs 50#.

For some reason, my dad doesnt think I am completely baked for picking up his yard everytime I go up there

[Phil Winkler]

Nice shots. Where does your Dad live in Ga?

[graveleye]

He lives in Kingston, Georgia. That would be about 60 miles NW of Atlanta, inbetween Cartersville and Rome. He has about 15 acres on top of a hill there. About 7 acres were cleared for a pasture, and the rest is old growth forest. Nice and peaceful.

[bigalpha]

graveleye -- they all look like chert/limestone. Hard to tell without either dropping HCl on it or doing a scratch test. I would say that they are just Chert or Limestone. That's my undergrad, almost graduated opinion.

[hewhocaves]

pretty quartz!

It's been awhile since I've had to dig up my Petrology notes on microcrystaline quartz and the like, but yeah, it looks very much like you have some quartz there. There may be ls in there, and there may not. Difficult to tell.

Properly cut and polished, the basketball sized one might make for good bookends. (i loved playing with the rock-saw)

[graveleye]

I'm actually looking into buying a rock-saw sometimes soon. I've really been into the lapidary hobby over the past few years.

I agree that there is limestone underneath my dads property. The hilltop he lives on is like a plateau.. flat on top, steeply sloping on the sides. Walk down the hill and you dont see these rocks.

Chert I could agree with, but its definatly not limestone as light passes through and if they are thin enough you can see inside the stone. The examples I have were scrubbed pretty good, but still have some grit in them. As well, it is much harder than any limestone I have ever encountered. I have some lumps of fossiliferous limestone that I got in Michigan. Its really pretty when polished, but you have to be careful to not run it too long or it wears away completely. Just for kicks I stuck two 1" pieces into the most recent grind I'm doing with these new rocks. Unfortunatly the limestone got completely eat up by the harder rocks. Oh well!

[hewhocaves]

it sounds like you have sandstone caprock (with quartz veining? over limestone. I'm sure someone's told you this before:

Go to the flanks of the hill and find the giagantic %$%@%#$ cave already!!!! lol

John

[Teresa]

Definitely banded chert. No question. I've got oodles of this stuff out of Mississippian Limestone in Missouri.

it is a 'rough' grade of agate--not gem quality but definitely pretty. Such a pink/blue banded chert is the Missouri state rock called mozarkite.

[graveleye]

I rather agree with you... its not agate quality, not insomuch as some I've been working with recently, but I still find it quite attractive, and its free.

So, how is banded chert formed?


John, the whole place is littered with small caves and rock-shelters. Not as many as there are when you really get into the far NW reaches of Georgia, but enough. When my dad had his well drilled they hit several voids, not that means much. They still had to go 300' before they hit water. But my brother says there is a shelter with a "crack" in the back that seems to go - breathes a little too. HA! So we set off down the hill and after about 20 feet I realized I am covered with ticks. Dads yard is the white-tail superhighway. You know how much I hate stinging sucking insects. I'm gonna wait now until after a hard freeze. That cave isnt going anywhere He says its not big enough to fit through, so if it looks like it goes, I am gonna need a digger.

[Phil Winkler]

From a Mo web site:

Some kinds of rocks are hard to get to know personally. Not so with chert. If your tire has ever been punctured by a chert sliver on an Ozark road, or if your air mattress has ever gone flat while camping on a gravel bar, you know what I mean.

Chert, also known as flint, is one of the most abundant rocks in Missouri, and is composed of one of the most common minerals on earth. In fact, in much of Missouri you could say that chert is "common as dirt."

Chert is a hard, fine-grained rock made up mostly of the mineral silica (SiO2). Silica itself contains the elements silicon and oxygen, two of the most abundant elements on earth. Silica is also the main ingredient in glass, obsidian, quartz, many kinds of sand and computer chips. It is a lesser component of numerous other rocks and minerals. Chert and flint are generic names for this siliceous rock, which comes in many colors and patterns.

Missouri chert is usually white, gray or tan, but almost any color is possible, depending on chemical variations in the stone and the details of its formation. Certain colors have their own special names. Jasper, for example, is reddish, orange or yellow chert. Agate is banded in many colors and is sometimes translucent, making it a favorite for jewelry and crafts. The Missouri General Assembly chose mozarkite, an attractive chert with a banded pattern of red, pink and gray, to be our state rock.

Most of Missouri is underlain by carbonate rocks-limestones and dolomites-that formed from sediments on the bottoms of ancient seas. This is the thick, layered rock we see along road cuts and river bluffs. Chert is associated with almost every type of limestone and dolomite in Missouri. It appears as globular-shaped "nodules" and "beads," and in pockets and veins of various sizes and shapes interbedded with the limestone.

Geologists are unsure exactly how the chert formed in the limestone. Two possibilities are that oceans deposited silica along with the calcium carbonate shells of marine life, and the chert formed into sedimentary rock along with limestone. Another theory holds that chert formed later, when silica-rich ground water permeated spaces in the limestone. Whatever its origin, chert exists in limestone, and chert nodules, veins and layers are easy to see in road cuts through chert-bearing carbonate rocks.

Chert is glasslike in its hardness and in its ability to withstand weathering. In fact, it is so much more resistant to breakdown than the surrounding carbonate rocks, that the chert fragments remain long after the limestone or dolomite has weathered away and developed into soil. This explains the abundant chert fragments in, and on top of, many Ozark soils.

Soil scientists classify and describe soils. They have given the name "Clarksville Very Cherty Silt Loam" to one of our common soils formed from cherty limestone. Clarksville soil is deep, but you wouldn't know by looking at it; the surface is mostly covered with rocks. But among the rocks there is some soil. From the surface down to 60 inches, Clarksville soil contains from 40 to 60 percent chert fragments. Profiles of cherty soils, including Clarksville, are exposed in road cuts throughout the Ozarks.

When rain erodes steep, rocky soils, some of the rock moves down the slopes and into streams and valley floors. Other rocks are redeposited within the streams. Finer soil material is carried away, leaving cobble and gravel-sized chert rocks in massive deposits in and along our creeks and rivers.

Those gravel bars that make such good boat landings and camping spots came from the chert that millions of years ago lay trapped in solid beds of limestone or dolomite. It took an unfathomable amount of time for these rocks to become free of the limestone so they could help form a gravel bar. But then, what's time to a rock?

Although chert usually is not thought of as a valuable mineral resource, chert gravel is used for surfacing roads in counties where the gravel is readily available. The chert is mined from deposits along the streams, providing an inexpensive source of road gravel. Chert gravel is also is used in asphalt in the construction industry. It is sold for landscaping as "river gravel." Mining directly from streams harms fish and other stream life, but there are ways to extract gravel from a stream's floodplain with minimal damage.

Chert once occupied a more critical status as a useful commodity: Stone Age people favored it for making tools. Hard, tough, fine-grained and brittle, chert can be fractured and chipped by a skilled craftsman to create intricate shapes and sharp edges. For thousands of years, chert knives, scrapers, spear and arrow points, drills, hoes, axes, grinding tools and ceremonial objects represented the peak of art and technology.

Some kinds of chert made better tools than others. Exposure and weathering changes the rock's appearance and tooling qualities. The chert rocks strewn over the ground and piled along streams are inferior for tools. Prehistoric Missourians dug open mines to secure the best chert for making tools. They also traded for special quality stone and stone tools, sometimes over long distances.

Today we find chert flakes in plowed fields that used to be Indian campsites. The abundant flakes are the non-biodegradable litter of thousands of years of tool makers. Occasionally, we may find pieces of the tools themselves and, more rarely, a perfect blade or point, hundreds or thousands of years old. Chert was also important for its ability to produce sparks which could start fires and later, to ignite gunpowder in flint-lock firearms.

In Missouri, chert usually appears in small fragments or narrow bands embedded in limestone and dolomite, or as loose rocks in the soil or along streams. But chert also can be found in sedimentary formations tens to hundreds of feet thick. These solid chert formations resulted from long accumulation of tiny marine animals and sponges that secrete glasslike particles of silica. A good example of this is the almost continuous layers of dense chert within the dolomite bluffs along some Ozark streams.

Some of these "chert reefs" are so persistant over wide areas that geologists use them as reference layers or "marker beds" to help them map the geological layers above and below the reefs. Primitive animals that once lived in these reefs sometimes show up in the rock as chert fossils, preserving the form of the once-living organisms. Even thicker beds of chert form the novaculite deposits in Arkansas' Ouachita Mountains. This special chert is mined to produce the famous "Arkansas stone" whetstones, which sharpen tools all over the world.

Missouri's most impressive beds of thick, solid chert are in the exposed cliffs and glades near Joplin and along Shoal Creek in Jasper and Newton counties. These unusual features are easy to see in Joplin's Wildcat Park and adjoining Wildcat Glade Natural Area on the city's southwest side. Wildcat Glade sports an interesting assemblage of plants-a mosaic of lichen-covered rock, patches of gnarled, stunted oaks and showy wildflowers, such as coreopsis, glade onion, rock pink, prickly pear and Barbara's buttons.

Grand Falls on Shoal Creek is only minutes from Wildcat Park and is perhaps Missouri's most scenic chert feature. Here, Shoal Creek plunges 15 feet over a ledge of solid chert to continue southward. In addition to its novel bedrock and great beauty, Grand Falls holds the Missouri record as our highest, continuously flowing falls. (Missouri has many higher waterfalls, but they don't flow all year.)